total loss of power at big throttle openings

[eagle747]

My 97 Vmax has just come out of rebuild. Had a lot of trouble with dirt in the carbs, so str4ipped them and used a rebuild kit - now got good smooth tickover, good pickup of power from low revs but.... at anything more than about 1/3 throttle opening the bike dies. Wont pull or accelerate and I can hear a change in engine noise that sounds just like fuel starvation.

So have stripped the carbs again, payting attention to the mains. All clean as a whistle. \Ran the bike with the filter off to check the CV pistons are moving etc.

SO at a loss now. Anyone come across these symptoms?
BTW have checked Vboost, put in a new fuel filter and checked the breather for blockage. All fine.
Now starting to suspect the fuel pump, CDI and vacuum advance, but cant really afford to replace on a 'just in case' basis.

Any ideas?

 

[maleko89]

What do your plugs look like? What are your float levels?

 

[jfeagins]

Air box re-fit snuggly back down on ALL carbs correctly?
Carbs pushed securely back down into ALL their boots correctly?
Air filter installed and airbox Y installed?

Is your fuel pump cycling properly?

 

[eagle747]

Plugs normal grey / brown. No sign of either soot or a weak mix - though not that surprising as the problem only appears at big throttle openings. Dont know about the float height - just looking in the Clymer manual - I guess I need a bit of clear tube to measure it. 15-17mm according to the book? Odd thing is all was ok before the rebuild and service, so wouldn't really expect float height to have changed. Just fitted new needle valves and it made no difference. Is it that critical?

 

[eagle747]

Air box re-fit snuggly back down on ALL carbs correctly?
Carbs pushed securely back down into ALL their boots correctly?
Air filter installed and airbox Y installed?

Is your fuel pump cycling properly?

Checked the airbox and inlet boots - seem fine. No sign of rubber cracking. Yes air filter in place - but what is the airbox Y?

Well the fuel pump is making its usual noises when the ignition is switched on but I guess that doesn't necessarily mean its working properly - but it is at least running - but I guess if it wasn't the bike wouldn't run at all.

 

[KosRider]

Did you sync the carbs ?? :ummm:

 

[one2dmax]

Get rid of the Keyster carb kits you probably used (if I am right the main culprit is the emulsion metering rod that the main jet threads into).

Sean

 

[eagle747]

Did you sync the carbs ?? :ummm:

Yes. All synced up nicely.

 

[eagle747]

Get rid of the Keyster carb kits you probably used (if I am right the main culprit is the emulsion metering rod that the main jet threads into).

Sean

Carbs are standard apart from 170 main jets (I guess they are Dynojet). None of that has been changed during the rebuild - and the problem wasn't there before. It has a K&N and a drilled airbox and Marving slashcuts - so I am assuming 170s are about right - but its what it was running with when I got her a year and a bit ago.

 

[one2dmax]

For mains you need 147.5's (about 155 dynojet) to get in the ball park. Also, put the stock needles back in if you've changed them. I'd also maybe swap out the plugs just in case they are starting to misfire (or not fire under the increased cylinder pressure when throttling up).

On the keyster problem I have pics of that in our facebook albums.

Sean

 

[maleko89]

Drain the carbs and check float level. That's the first thing that should be set.

 

[eagle747]

Thanks. I dont know if the needles are standard so may have to just replace them anyway. I thought standard main was 152.5? I am pretty sure this bike has been run on nitrous, having found the wiring and the threaded holes in the inlet stubs (now replaced). Its still very odd - as she was running fine prior to rebuild and nothing was done to the carbs.

 

[eagle747]

Drain the carbs and check float level. That's the first thing that should be set.

Carbs have been cleaned out. Not set the float heights - but then it was all working before. I guess I had better do that next.

 

[maleko89]

Carbs have been cleaned out. Not set the float heights - but then it was all working before. I guess I had better do that next.

I always check because the float tang is easily bendible. Same reason why you have to sync after removing carbs....anything can change.

 

[eagle747]

I always check because the float tang is easily bendible. Same reason why you have to sync after removing carbs....anything can change.

Yep - carbs will have to be re-synced as they have been off again. Once I get em back on I'll check the float levels.

I wouldn't have thought tho that this would have such a big effect - its not just stumbling a bit - its like the ignition has been turned off! Then I back off the throttle a bit and all is well!

 

[maleko89]

Also make sure rubber caps at bottom of each jet block are in place and tight.

 

[eagle747]

Mark
Yes - done that - caps are fine. Running out of ideas. I have done a sort of test on the fuel pump - thumb over the pipe. It seems to have decent pressure so its not that.

 

[eagle747]

Float levels are within spec. I have discovered tho that some of the clips that go round the rubber carb boots are knackered and dont tighten (I didnt put the bike back together myself) - so air may be being sucked in. Off to automotive suppliers for some new clips....

 

[rbolyard]

Trust Sean on this. He found Keyster parts on my 85 and after the carb rebuild it now runs fantastic.

 

[eagle747]

I promise I really dont have any Keyster parts in my carbs! I am not even sure we can get them in the UK!