68 cobra float bowl gaskets

[Ivan]

I would do the 750's, but someone needs to give me a donor bike... I don't have one.

Ivan

[Vintageman]

Ian,

When you make that graph you start low rpm and crack throttle wide open, hold it there, until it reaches max rpms. So really it’s just the main jet circuit being graphed?

I know from your other posts you tune all throttle position circuits that a Mikuni provides. How to you graph/test that to know a given circuit is better?

For example, I try to do the same thing you see people on dyno videos, but instead of wide open I would try all the positions each help steady fixed and let it rev up to see how it behaves throughout power range. That is more how one drives on the street in a mildly aggressive fashion if I can coin the style. By mildly I mean changing to a fixed throttle position well under WOT and hold it, by aggressive I mean letting the bike a rev a ways into the power band versus shifting early in rpms

I have found my 75 T500 very very tough to tune perfectly everywhere with the Mikuni circuits provided and using stock needle jet and jet needle only That is changing, clip position (even half position), pilot jet and main jet size only. Same true with round slide only the stock one I have or can find

Here is what I see (funny analogous to that graph but different metrics).

Stock: Pilot a little a lean. Can be fixed with pilot jet OK,
Stock: Main lean and can be fixed by bigger main jet ok
Stock Needle Jet /Jet needle not good at all:
The challenge is the ¼ and ¾ range either too rich or too lean… The main jet can help ¾ and pilot can help ¼, but trade off is you compromise their main contribution.

Much more sensitive with my chambers (jemcos) on bike

You think you have it than temperature changes by 15 F and problems in one of those spots again.

Did you see something similar I wonder?

More T500 info:
This challenge is with my 75 T500 as said. Later, I changed the cylinders to the early T500 ones. They have a lower intake floor. I run the later T500 carbs still and inatake items.
The jetting was real close and the only difference was maybe was the temp from last time I tuned it, not sure, but not too much difference with cylinder change only for sure.

I have the early carbs, intake boot, and air box, but for stock appearance/value I did not change. Also another gent on this board did not see any difference with early or late air box. I can verify that for the GT250 early and late made similar change to airbox (dame alter air boc onwith t500). I tried both and ended up with same main jet for either no gain in power as I hopped using early versus late air box.

Also confident I got most of the power back that was lost when Suz changed to later t500. I know the early t500 intake boot makes carbs closer to cylinder and carbs tipped to remove some kick in flow. But, I am very happy with the improvement just cylinders.

I also feel there is a reason those early carb venting design got dropped. For example if I change intake boot and air box, I would still use later carbs

If you ever looked at early t500 rubber intake interface to cylinder you will see that it isn’t that great a flow path alignmentand there is a kink/lip. I think the later intake (Al part) is smoother fit even if longer and carbs level. Still we are talking peak power well under 8K rpm and don’t believe a tad shorter length adds a bean of power…. Maybe worse for longer better resonance tuned? Maybe wrong if real bored will try

Well this info of how I changed my bike is irrelevant. , say it for I think the jetting difference is more how carbs are vented.

I am interested in your kit for later T500 carb, but would like to know in general what you changed to see if it will improve my set up with chambers. I would like to for once tune that bike well centered and then it has enough margin to withstand a 15-20F temp change without significant gurgling from too rich or flat/pinging for too lean

Also any plans for the early GT550?
The lower throttle positions suck. Very small opening too rich when a little more position OK or visa-versa (new slide cut if I could find one). If no plans any advice?

[Ivan]

Ivan,

When you make that graph you start low rpm and crack throttle wide open, hold it there, until it reaches max rpms. So really it’s just the main jet circuit being graphed?

This is correct but includes the needle length and tip diameter.

I know from your other posts you tune all throttle position circuits that a Mikuni provides. How to you graph/test that to know a given circuit is better?
Exhaust gas tests prove everything

For example, I try to do the same thing you see people on dyno videos, but instead of wide open I would try all the positions each help steady fixed and let it rev up to see how it behaves throughout power range. That is more how one drives on the street in a mildly aggressive fashion if I can coin the style. By mildly I mean changing to a fixed throttle position well under WOT and hold it, by aggressive I mean letting the bike a rev a ways into the power band versus shifting early in rpms

This engine will easily run for the red zone with 1/8th throttle in the lower gears (on the road) with the carbs set up the way that they are.... and transition from one circuit to another seamlessly that you cannot feel anything.... cruising at light throttle has much less intake noise due to the proper relationship between the pilot/needle/needle jet.
The amount of throttle to keep the bike moving is considerably less than with stock jetting.

I have found my 75 T500 very very tough to tune perfectly everywhere with the Mikuni circuits provided and using stock needle jet and jet needle only That is changing, clip position (even half position), pilot jet and main jet size only. Same true with round slide only the stock one I have or can find

That's because the stock parts are junk... a waste of time that cannot ever work properly by my standards.... needle tapers are wrong, needle jet is the wrong size...

Here is what I see (funny analogous to that graph but different metrics).

Stock: Pilot a little a lean. Can be fixed with pilot jet OK,
Stock: Main lean and can be fixed by bigger main jet ok
Stock Needle Jet /Jet needle not good at all:
The challenge is the ¼ and ¾ range either too rich or too lean… The main jet can help ¾ and pilot can help ¼, but trade off is you compromise their main contribution.

Much more sensitive with my chambers (jemcos) on bike

You think you have it than temperature changes by 15 F and problems in one of those spots again.

This is because the stock parts are delivering the wrong mixture... the chambers will exaggerate the existing problems. When the jetting is correct at all the throttle positions, it won't be sensitive at all.

Did you see something similar I wonder?

More T500 info:
This challenge is with my 75 T500 as said. Later, I changed the cylinders to the early T500 ones. They have a lower intake floor. I run the later T500 carbs still and inatake items.
The jetting was real close and the only difference was maybe was the temp from last time I tuned it, not sure, but not too much difference with cylinder change only for sure.

I am not concerned with the porting differences...(at most this will be 1 main jet size if that)
The later boot and airbox makes a major difference to the main jet... I saw this on the dyno yesterday when I just swapped the later boot w/internal V-stacks for the stock early boot... it drowned in fuel by at least 4 main jet sizes.... maybe more.

I have the early carbs, intake boot, and air box, but for stock appearance/value I did not change. Also another gent on this board did not see any difference with early or late air box. I can verify that for the GT250 early and late made similar change to airbox (dame alter air boc onwith t500). I tried both and ended up with same main jet for either no gain in power as I hopped using early versus late air box.

Also confident I got most of the power back that was lost when Suz changed to later t500. I know the early t500 intake boot makes carbs closer to cylinder and carbs tipped to remove some kick in flow. But, I am very happy with the improvement just cylinders.

I also feel there is a reason those early carb venting design got dropped. For example if I change intake boot and air box, I would still use later carbs

If you ever looked at early t500 rubber intake interface to cylinder you will see that it isn’t that great a flow path alignment and there is a kink/lip. I think the later intake (Al part) is smoother fit even if longer and carbs level. Still we are talking peak power well under 8K rpm and don’t believe a tad shorter length adds a bean of power…. Maybe worse for longer better resonance tuned? Maybe wrong if real bored will try

Well this info of how I changed my bike is irrelevant. , say it for I think the jetting difference is more how carbs are vented.

I am interested in your kit for later T500 carb, but would like to know in general what you changed to see if it will improve my set up with chambers. I would like to for once tune that bike well centered and then it has enough margin to withstand a 15-20F temp change without significant gurgling from too rich or flat/pinging for too lean
There are many things wrong here... needle jet wrong size, needle taper starts too late, 2nd taper way too steep, end dimension too small, main jet too small. It is not possible to get everything right with the stock parts.
None of the kits that I sell are sensitive... it's sensitive because there are many things wrong.

Also any plans for the early GT550?
The lower throttle positions suck. Very small opening too rich when a little more position OK or visa-versa (new slide cut if I could find one). If no plans any advice?

Once again, I would work on something, but I need a bike... This also has to be done during the winter... This winter I have 6 ECU reprogramming R&D projects that I need to have ready by spring... the T500 is getting the spare moments this winter to hopefully be finished by spring.

Getting the small openings correct is quite a challenge...

The changes to the 34mm carbs are quite a few:

Convert from internal vent to external by blocking the original passage and making a new one and installing the reproduction brass vent tubes.
(photo instructions are included with the kit and all special fittings, hardware and tools)

New needle jets custom sized reproductions - brass

New 4 angle needles - mirror polished stainless steel for wear consideration

New needle and seat assy's (3.0) similar to the later style w/spring loaded float pins for more consistent fuel level with minimal wear.

The 69-72 32mm carbs have many of the same problems that the 34's had... just not as bad.

The late carbs have the same problems that the H2 750 had.. they will need the least amount of work.. probably just needles/needle jets... I'll know when I get there.


Ivan


Here's a chart that compares a 75 T500 to the 68... The long intake system helps a lot at low rpm... flat up top. 3 hp is a lot on these old bikes.... (10%) You can easily feel it as it tachs up.


....

[tz375]

Ivan,
That's another eye opening graph. Thanks for that.

Was that last graph a complete early 34mm carb set up on a 68 motor versus a complete later model bike with 32mm carbs or was it the same carbs and motor with different intakes?

[Ivan]

The 75 is a complete 75.... not my bike.

My bike is a 68... all 68

The chart shows the most predominant difference... intake length.

The porting differences are a lot less significant (a few percent -maybe 2-3)


Ivan

[tz375]

Got it. Thanks. That's a relatively huge difference in the way that the curve is rocked. I have seen that sort of change with big differences in carb diameter but that's the first time I have seen intake length make so much difference.

Makes me wonder about the long intakes on my GT750 and what the curve might look like with really short intakes for the strip where it's all about top end versus street where it's the opposite.

In your experience is that typical or is it more specific to T500s?

[Ivan]

It's pretty much expected because of the huge difference in intake length.... the shorter duration intake port helps here too especially below 3000.

But, on the early vs. late T500 intake length, they have added appx 3 inches for the aluminum port adapters, another 2 inches or so for the boots, and another 4-5 inches for the airbox boot's internal stacks... that's a lot. (appx 8-10 inches)

I made a intake length test while dyno testing my 1974 380.... by removing the boot (internal stacks appx 5 inches +/-) and adding a larger main with foam pod filters.... The difference was similar to the chart above.
In the 380's case, it gained nothing on the top end, probably due to not enough exhaust duration to take advantage of the shorter length.... but the bottom end was destroyed by comparison below 6000.
I'm not racing it so further investigation wasn't a high priority.

In order to use the later airbox boot on the T500 the air filter needs to be spaced downward and the main jet reduced a few sizes... I may investigate this further to see if there are any low rpm advantages with the early airbox and later boot combination.... probably due to the long stroke and low rpm range that this engine operates in, the proper length had to be made that long (later T500) in order to make a noticeable difference.

Maybe your 2 stroke software can calculate this... My guess is that they kept lengthing it until they got the most desirable effect from it... when they ran out of room, they put the stacks inside the airbox boot.


Ivan

[Ivan]

TZ and others,

Here's a couple of charts of my GT380...

The bottom chart is adding uni foam filters instead of the airbox (getting rid of the internal stacks) and raising the main jet to suit

Ivan


...

[Vintageman]

Love this site!

Ivan,

You stated “Exhaust gas tests prove everything"
what is the theory behind exhaust gas as a criterion?

Correct me if I am wrong, that data is still just for that WOT (main jet plus needle position to needle jet office at that throttle point) : Not other throttle positions.

If WOT test only, than you must have run tests trying for example ¼ throttle position or ¾ throttle position also?

Or are you saying WOT graph covered them too? confused.

If I run my 75 T500 WOT and pick a main jet that gives best result for the given location then if all I ever do when I ride is crack WOT no worries. Not typical for a street rider to always ride like that. I would have never known my issues with needle taper (I agree with your results about needle) unless I used those position that expose them for example ¼ or ¾ throttle. And even here, issues when just a little off, may only show up at certain RPMs.

Clarification please
Are you saying you just tried that 1975 T500 and all you did was change the Y Boot and air box with the early type and then had to drop main jet size quite a bit? You did not change carb to cylinder boot?

Did you get more power everywhere or did it rotate the power band?

I always take those air horns out. Sure I did on my 75 T500 too (will check). On my 75 T500 I am running several steps above main jet called out for a stock 75 T500. But, it was the cylinder swap with early 500 that woke up my 75 T500 last season

Changes in design between 68 500, 69 T500 and 75 T500 that can result in why the graphs are different.

Design changes are.
1) Intake Port timing
2) Intake manifold (length and carb angle)
3) Carb (OK 68 is a 34mm others 32)
4) Y boot
5) Air Box

Graph observation.
a) Definite pivot point at about 5500 RPM.
i) 75 has more bottom end
ii) 68 has more top end
b) No real difference in max RPM

Ivan you say 2) 4) and 5) are more dominant than 1) for the T500 line

So On my 75 I did 4) (by ripping out air horns) than 1) so far. I say (no proof) that pivot and power boost was much more significant for me when I did 1). I'll try 2) and 5)

Ivan,
When you sell the kit will you sell the early and late T500 separately or if similar Jet Needle for both for example will you combine as one kit?

Thanks

[tz375]

Ivan,

I took one set of MOTA data and simply stretched the intake length a bit and in that theoretical case the long inlet increased low end by 1-2 hp and the short was better at the top end by a similar amount. Not 110% accurate but supports your real world findings. When I changed it to an excessively long inlet, it lost 4-5 HP at the top end and added NO extra bottom end.

34mm inlets (carbs) versus 32mm was worth 0.5 - 1hp at peak revs and after the peak but little difference lower down. Very interesting.

Just out of idle curiosity, I did a similar experiment with GT750 data and the difference between stacks (long inlet) and pods or open inlets was almost 2 hp at the top end and 2 HP less low down with a tipping point of 5500. That tells me that my race stacks are better suited to the road and my UNI filters for the street might work better at the track. Oddly enough my highest trap speed came with pods on which tends to confirm that theory.

OK, so now I need stacks inside pods for the street and super short ie zero length bellmouths at the track. Thanks. That confirms why Lectrons on drag bikes have ultra short bellmouths. The more I learn, the more I realize that I don't know.....

Vintageman:
Exhaust gas analysis in terms or A:F or unburned HC, or CO or CO2 provide an indication of the completeness of combustion. Some dynos just offer an Air:Fuel number and it's easy to see at any RPM or throttle position whether the mixture is rich or lean. Others offer more data with 5 gas analysis including NOx.

With a TPS, it's easier to plot out the 3 dimensional data, but it's not hard to watch the monitor to see whether the mixture is rich or lean at different throttle positions and it's fairly easy to feel the changes in roll on response. I can't speak to what Ivan has available to him, but he should be able to measure A:F at whatever revs and TPS he wants and can measure or feel he difference in pick up at any position.

[tz375]

Ivan,
I have a couple of sets of VM32 OEM zinc bodied GT750 carbs in front of me and the low top is marked 310-12 and the high top is 310-13 and the bodies appear to be identical, apart from a minor casting difference.

I compared them to photos of a couple of T/GT500 carbs I had in the shop a couple of years back. The 500 carbs were a 32mm carb (153-13) and 34mm (150-10) from a Cobra.

Both 750 carbs are very similar to a T500 32mm carb (153-13) in that they look like they are designed to be homo pressure, but only the 68 Cobra is vented into the bellmouth. The others actually vent the bowl to atmosphere above the venturi. That suggests that the T500 carb I had was a later type.

It also suggests that the same body was used in GT500 and J and K model GT750 but were stamped differently to recognize jetting and slide differences. It also suggests that owners of T and GT500s need to be careful not to confuse the carb bodies if early and late 32mm carbs are vented differently. Although they look very similar, they are not the same.

[Suzukidave]

Richard , when you were asking about short intake i was thinking you were asking about the the intake between the carb and the cylinder . Years ago just to try it i replaced the stock rubber piece between the carb and cylinder with a piece of radiator pipe that put the carbs butted right up to the cylinder and i was suprised how much perkier the engine ran

[tz375]

Good point Dave,

Pressure waves travel from the piston to the open end of the duct. With a decent inlet design, that all the way back up in the air box.

Making the change to the section between carb and piston is a sure fire way to change inlet length. Making changes on the upstream side of the carb may have slightly less impact because of changes in section through the carb. When I ran the MOTA simulations, I chose to vary the length on the airbox side just to see what happened and it predicts a response. That appears to be consistent with what Ivan is seeing on the dyno.

[Ivan]

Love this site! What exactly are you referring to here?

Ivan,

You stated “Exhaust gas tests prove everything"
what is the theory behind exhaust gas as a criterion?

I use a CO tester on the old 2 strokes and most other engines that I tune... CO% is the most relevant gas when tuning... I have found this to be true after 25 years of doing this for a living.
I test all throttle positions throughout the rpm range... I do not disclose my gas readings as I consider them proprietary info.

Correct me if I am wrong, that data is still just for that WOT (main jet plus needle position to needle jet office at that throttle point) : Not other throttle positions.
The above dyno tests are just showing full throttle

If WOT test only, than you must have run tests trying for example ¼ throttle position or ¾ throttle position also?
Not recorded on the dyno... Just exhaust gas tests (these are not recorded) the same way that I have always done it for every other kit that I have made in the past.

Or are you saying WOT graph covered them too? confused.

If I run my 75 T500 WOT and pick a main jet that gives best result for the given location then if all I ever do when I ride is crack WOT no worries. Not typical for a street rider to always ride like that.
Nobody rides like that except a drag racer....

I would have never known my issues with needle taper (I agree with your results about needle) unless I used those position that expose them for example ¼ or ¾ throttle. And even here, issues when just a little off, may only show up at certain RPMs.

Clarification please
Are you saying you just tried that 1975 T500 and all you did was change the Y Boot and air box with the early type and then had to drop main jet size quite a bit? You did not change carb to cylinder boot?
The dyno chart with the 75 T500 is a 100% stock T500 vs. my 68 with my jet kit...

I installed a late model airbox boot onto my 68 and it drowned in fuel.... this is not on any of the charts. The point here is that it's restrictive and contributes to a smaller main jet.... it doesn't mean less HP... just increases fuel flow from the carb. There can be some intake restriction and still deliver full power, but main jet size will be different.

Did you get more power everywhere or did it rotate the power band? I did not try to rejet it with that boot.

I always take those air horns out.
Sure I did on my 75 T500 too (will check). On my 75 T500 I am running several steps above main jet called out for a stock 75 T500.
This would make sense because you decreased restriction... this doesn't mean that you make more power... the ass dyno doesn't work as well as a real one.

But, it was the cylinder swap with early 500 that woke up my 75 T500 last season....
I can't comment on this because I don't have late cylinders on my 68.... but testing on other bikes with minor porting changes doesn't have a major effect on jet sizes like intake restriction does. I also do my own testing to prove things out.
Raising the roof of the exhaust port 2mm and lowering the intake 4mm on my 75 H2 made no difference to the jet size on that bike as compared to the stock cylinders.

Changes in design between 68 500, 69 T500 and 75 T500 that can result in why the graphs are different.
I've done my research here.... I am aware of the things that were changed.

Design changes are.
1) Intake Port timing
2) Intake manifold (length and carb angle)
3) Carb (OK 68 is a 34mm others 32)
4) Y boot
5) Air Box

Graph observation.
a) Definite pivot point at about 5500 RPM.
i) 75 has more bottom end
ii) 68 has more top end
b) No real difference in max RPM

Ivan you say 2) 4) and 5) are more dominant than 1) for the T500 line
color=#0000FF]That's correct for any engine that i have tuned to this date.[/color]

So On my 75 I did 4) (by ripping out air horns) than 1) so far. I say (no proof) that pivot and power boost was much more significant for me when I did 1). I'll try 2) and 5)

Ivan,
When you sell the kit will you sell the early and late T500 separately or if similar Jet Needle for both for example will you combine as one kit?

I would like to try to sell the 32mm as one kit with specific instructions for both types of venting, but I'll see how it goes and what's actually needed because I haven't installed them yet or tested anything with them.

Thanks

[Vintageman]

Ivan,

I enjoy this forum for everyone takes the time to explain things

Here's hov a hobbyist checks for most efficient fuel burn (a.k.a exhaust chemistry).
1) Best MPG
2) Have someone ride behind you and see hov long before he or she passes out from fumes.

I agree, just because you make air inlet less restrictive does not mean you alvays get a huge increase in pover due to rpm range these stock bikes have. For sure you get a lot more intake noise and then need to adjust 3/4 to full throttle (main jet may do it if all for you if needle taper good fit tovards the end)

I knov most all of knov the difference betveen the T500 bikes, I listed for just so far that intake porting timing had the most difference for me in pover and its shape and not having to change jetting for that change for it to run decent

The needle jet is vrong for the T500, no other needle on market is close enough or has right changes as you found are needed. I think different slide cut (richer) could help but NA too

I am interested in your kit vhen released and glad you tyr to cover both at once for guys vho mix and match years like me.
You have done all the vork and that is significant as you noted!

Do you make your on Needle Jet from scratch and/or modify existing ones?
There are are several Needle Jets series obsolete. This is devistating. If you could reproduce these I think there is a market.

Thanks

Mike S