Cannot rev past 6500 RPM under load

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n00bzi11a

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So, I'm a brand new European member but I've roamed around this forum and still I'm unable to fix my bike:

I've bought an cursed 85 restricted French Vmax, site unseen, and worst of all, sent a buddy of mine to check and bring the bike (since he was a veteran rider and I've assumed he would notice if something was not okay with the bike... Boy was I wrong about that...

Anyway, after fixing a bunch of things (replaced the Clutch, DD Clutch spring mod, replaced oil seal between driveshaft and gearbox, oil seal ou the final drive, a couple of electrical connections, and so on...) my final and main issue is that the bike has absolutly no power past 6500 RPM. cannot even rev past that from third, fourth and fifth gear. When in neutral however it revs just fine.

What I've done to try to solve this mistery:

- Check all four spark plug coils with an multimeter, after replacing the 4 cables, everything seems fine and the resistence for every coil is according to the Haynes Manual, teher are no cracks or anything. Also disassembled and cleaned the spark plug boots, and all 4 sparkplugs have a strong spark.

- Tried to unplug the electrical connector to the Small advance timing advance BOX (TCI ?) Whith the vaccum still attached to avoid a leak, no change. Also I've mesured it with and without vaccum applied and it indeed its voltage varies with the applied vaccum (as according to Haynes)

Replaced the fuel filter as it was full of junk (the fuel tank as some rust inside...)

Now... Onto the Carbs:

-I've replaced all 4 diaphragms and pistons ( 3 out of 4 had very tiny holes and when replacing the diaphragms, one of them srumbled on my hands so I've replaced the pistons wich ones from UK Ebay, they are aluminum instead of plastic, slide very well and are the same weight as the originals. Still no improvements to Power (Note: The small oring on the diaphragm covers are there and in good shape.)

- Opened all four carbs bowls and cleaned everything inside. No blockages or gunk found. Still no improvements.

- Adjusted the Needles to both ends and got no improvements,

- Adjusted the bowl float, which at the beggining had a wet level of 22mm... Made it go to the rich side (maybe too much) at 14 mm... still the same... But now I have fuel running from the needle jet "hole" of the left carbs (the rear one is worst) when running on the sidestand. Changed it already to the 28.5mm dry level (or 1.120" as mentioned here on the forum a couple times), need to check the wet level as well as the carbs are outside the bike at the moment.

-With the very rich fload adjustment, tired to run a little bit without air filter... Also no changes

Every jet seems to be the correct size, but since the bike runs very strong on the low end and only has issues at te top end, I'm tending to be suspicious about the main jet, which is 152.5 on all four carbs and looks like to be mikuni jets.

Judging by the small plugs on the main jet holder (which are in very good shape, flexible and soft) I'me assuming the carbs were rebuilt by the previous owner... but I'm not sure. Also the gasket between the carb and jet holder in in very good shape, but there is only an oring around the emulsion tube. I think that should be another oring in one of the channels but it doesn't exist in any of the carbs.

The Carb boots are quite cracked on the ends, I don't think the cracks go into the midle and cause a vaccum leak But a can be wrong... Propane torch didnt show any major leak. I've also noticed that even when fully seated and thightned, I can remove the airbox quite easly by just pulling it... Airbox is stock and running with or whtout the Y does not improve the power issue.

Despite being a restricted model (whitout vboost) the bike does have what I can describe as stage 7 vboost tubes, which were plugged when I bought the bike. Opening them only made the idle quite weird and no improvement on the top end.

In a final Note: I didn't chage the spark plugs or the air filter because to be honest does not seem to be the culprit.

The carbs were not syncrhonized (only measured with an caliper to be on the ball park and not all over the place) since I don't have an vaccum tool. But since the issue is on the top end at WOT, i think if they are a bit out of sync does not make any difference.

I didn't made an compression test... But the bike runs and pulls very well at low to mid rpm... up to 6K and seems to hit a redline at 6.5K or 7K RPM. The sound when this happens also kind of remins hitting a rev limiter, and the bike also stutters.

Other than tve open Vboost Tubes, the bike seems to be stock

Another issue that can or cannot be related. I don't seem to have a fuctioning fuel or oil light. When I hit the red kill switch, sometime the bike takes a while to shutdown (i think the relay or whatever is called, under the left scoop of the bike is acting weird) and only when the kill switch is on the off position (whether the engine still remains running or not) i get a slight slow glow from both oil and fuel lights.

At the moment is everything that I remember from the top of my head. Any suggestion or help is appreciated.

Best Regards.
 

Fire-medic

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Use these items to help set things right.

VMax carb float area.png VMax carb floatlevel bowl off.jpg VMax carb kit K-L labelled.png VMax carb kit.jpg

Mikuni 152.5 sounds correct, but I would re-install the VBoost block-off plugs. I also would find the coin to buy a synch tool. Carbs need to be synched to work well.

Sounds like you used a tank of propane to check the boots for air leaks. Try starting fluid and don't forget the bases of the intakes where they bolt to the cyl heads. There are thin O-rings there, and if you find one is leaking, remove/replace all 4 O-rings. The boots for the airbox or the donuts from the carbs to the VBoost manifold, or whatever you have, develop superficial cracks which often do nothing detrimental.

If you can pull off the airbox easily, with the carb clamps being fully-tightened, I would probably replace the rubber donuts from the airbox to the carburetor bellmouths. The clamps have a stop which prevents over-tightening.
 
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n00bzi11a

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Use these items to help set things right.

View attachment 86598 View attachment 86599 View attachment 86600 View attachment 86601

Mikuni 152.5 sounds correct, but I would re-install the VBoost block-off plugs. I also would find the coin to buy a synch tool. Carbs need to be synched to work well.

Sounds like you used a tank of propane to check the boots for air leaks. Try starting fluid and don't forget the bases of the intakes where they bolt to the cyl heads. There are thin O-rings there, and if you find one is leaking, remove/replace all 4 O-rings. The boots for the airbox or the donuts from the carbs to the VBoost manifold, or whatever you have, develop superficial cracks which often do nothing detrimental.

If you can pull off the airbox easily, with the carb clamps being fully-tightened, I would probably replace the rubber donuts from the airbox to the carburetor bellmouths. The clamps have a stop which prevents over-tightening.
Thanks you for your sugestions.

The carbs floats were ajusted using the second picture as a reference for dry float level.

I don't have any starter fluid at the moment. Would brake cleaner work as well?


The Orings on the intake seem fine and most importantly are installed.

Regarding the air box. I've noticed the clamp stops and insisted quite a bit with the propane tank after reading that someone tried to run without the air box and the bike would not rev past 6k or something.
 

n00bzi11a

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Have you checked that the vacuum advance is operating correctly?
Yes.

I've removed it from the bike, and followed the instructions from the Haynes manual (applying 12v to certain contacts and measuring the 2V signal from a diferente contact, and the 2V would change based on the vacuum applied). It worked as expected but I can tell that it was replaced in the past because someone wrote with an white marker "Vmax 1200" (like it was from the junkyard or something).
Maybe the previous Owner was after the issue as well?
 
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