Hi i've been looking thru this site looking at gettin some info on why my friends max is running so poor and have a question, I see some of you run with the boost valve fully open all the time, does this not cause overrichness at lower speeds ? or do you down jet on the pilot circuits to compensate ? thanks in advance for any comments
Help Support VMAX Forum:
sdt354
Well-Known Member
To start,what exhaust system is your friend using? If stock, connect the vboost back up.
Steve-o
Steve-o
tothemax93
Well-Known Member
You have to get more info on your friends bike. Does it have an aftermarket exhaust or stock. Does it have any type of jet kit. Is your friend v-boost open all the time now. It Could be from poor tuning or it could also just be dirty carbs. Do a search on "pea shooter" or "Shotgun". These are a couple of methods for cleaning up the carbs. I'm sure others will chime in also.
its a max thats been triked,it did have 4 straight pipes and i advised him to get some mufflers on it,which he did,he had a local guy turn them into 4-2 with some,I think gsxr cans on it,I've been thru the carbs,has new diapragms,needles and needle jets,new gaskets and seals all yamaha,jetting is stock for the year,new air filter. I did also find two of the choke plungers were sticking partially open and thought that may be it. checked all the coils,leads and caps,fitted new plugs,it ran the same ! boost servo checked out ok as did vacuum advance.drained float chambers as i didnt have any clear tube and measured them,left side carbs,of which cylinders seem to run cooler, had two different levels and right side were pretty much the same.I had a spare fuel pump so I fitted that and checked fuel capacity again and this improved things but I checked them 3 times and they varied all over the place ! pulled carbs and checked servo cable adjustment and found that the left side butterflys were not closed fully,redid the cable adjustment and i can get them to close but now they dont open fully when you do the key on thing. Am gonna go thru the carbs again now i read that the pilot jet in the block is very small and recheck they are fully clean,also gonna have a look at the float levels and do a wet check on the bench.I also have a spare set of pickups as I cant seem to find a decent test procedure for them,conflicting values and all that,thanks for your input guys
Oh I also checked valve clearances and cam timing,all good and compression is in spec, it has new battery,is charging ok,starts right up from cold with the choke,revs fine thru the rev range at standstill,(which the owner says sounds better than it did before it came to me),servo starts to open at about 5800-6000 but try to ride it and if you thrash it goes like it should,but driven normally it seems to have no power at lower rev range
sdt354
Well-Known Member
The V-boost open all the time is not likely to be a problem or require jettiing to compensate for it on the low end. That's if, the exhaust flows as well as a typical after market full system.
The main jets are usually down sized with a stock air box and full exhaust system. Pilot circuits are typically richened a bit over the factory settings on stock and modified engines.
Steve-o
Most of what you describe sounds, to me, like jetting issues due to the after market exhaust set up, but..
This sounds like a synchronization issue. Synching these carbs can make a world of difference down low, and right off idle. Have they been synched since the last rebuild? And they will need re-synched again, if you tear them apart again.
This sounds like a synchronization issue. Synching these carbs can make a world of difference down low, and right off idle. Have they been synched since the last rebuild? And they will need re-synched again, if you tear them apart again.
I havnt had chance to look at it yet as I had an accident at work and damaged my hand,carbs were not syncd with gauges but visually the butterflys looked ok,I would have thought good enough to run reasonably well,are the carbs that finicky on these bikes ? I'll have to borrow a set of guages and will do them when I refit,I havnt had to balance carbs for donkeys years as I normally only work on Harleys !
tothemax93
Well-Known Member
Yea, the carbs are that finicky
Ok,finally got around to going through the carbs again,all jet / airways clear and good.checked the float levels with a dial caliper and found the cast mark on left side carbs to be 2mm off, reset them all to 16.8 mm from the jet block,refitted them late last night,started the bike and it was noticebly better,all pipes warmed up the same,didnt synch them as it was so late and I had work today.Owner got all excited when I told him today and he was waiting for me when I got home,he took it straight out for a ride and returned and I quote said "I think I've come in my pants" It rides so much better and hits the boost much quicker when you gas it,didn't synch the carbs tonight as I had other work to attend to,aiming to do them either tomorrow or saturday morning.Seems that it was a combination of,low fuel levels on 1 +2, boost valves not closing properly and possibly the fuel pump,as the one I fitted seems to have a bit more pressure to it.Thanks those who commented for your input,will update when the carbs are synched,but I think its pretty much cured.
tothemax93
Well-Known Member
Great to hear you resolved it :clapping:. depending on how far the synch is off, it might even get considerably better.
twistedmax
Well-Known Member
I know it seems obvious but you didn't mention anything about the spark plugs, how do they look, have they been replaced?
Plugs are new,and initially #1 + 2 looked a bit richer than 3+4,but not wet,bike is a lot better now and going to synch the carbs as soon as owner is here to take it out and warm motor up,as I'm not insured to ride it
Well we synchd the carbs today,they were pretty much spot on,just a small adjustment needed, Owner is well happy and so am I. Glad I can finish the thread with a positive ending,nothing worse than reading one and coming to a dead end ! Thanks to those here that offered advice
