motor breaks up on top end

Ok I have a mild built 390 clone with a 34mm Mikuni square slide motorcycle carb on alky and it breaks up and pops threw carb wont rev up clear any idea,s on what my problem is ???????????
 

astrowrench

Member
Valve lash,timing,leaning out; take your pick; has to be one if valve float is not the problem; as it doesn't sound like it the way you describe it; what does plug look like??? Head Temp?
 

Kart43

Member
Your title says "Breaks up on top end" your description says "won't rev up clear" big difference here, if it is breaking down at 600 RPM that is one thing, but if it is spitting and sputtering up to 3K, 4K, 5k, this could many other things. Clarify how your engine is working it will help.
 
Your title says "Breaks up on top end" your description says "won't rev up clear" big difference here, if it is breaking down at 600 RPM that is one thing, but if it is spitting and sputtering up to 3K, 4K, 5k, this could many other things. Clarify how your engine is working it will help.

it revs up pretty far then starts popping and and breaking up I got it to rev up pretty good but not every time
Timing is set pretty close to stock
 

Kart43

Member
There are dozens of slide mikuni motorcycle carbs out there. It will be hard for anyone to tell you what jet and needle you should be using unless they have tuned the same carb to the 390. If your carb has a needle with an adjustable clip, I would move the needle up by lowering the clip on the needle. It sounds as if you are too lean.
 

flattop1

Dawg 89
it revs up pretty far then starts popping and and breaking up I got it to rev up pretty good but not every time
Timing is set pretty close to stock
I am with kart 43 on this though was going to say rich. either way if lean does not help go the other way.
Is this under load?
 
Ok so heres whats its doing under a load everything is good until its all the way open then it sputters between 3/4 and and wide open I raced it today and yesterday and Iam not sure where the problem is
Iam thinking 1 of 3 things or all 3

1st I haven,t match ported intake and head its close but not perfect ( i have to borrow a dremel )
2nd its stock size valves and carb is too big
3rd the rockers that Iam using are the NR racing billet steel rocker that it says they,re 1.23 ratio


Any idea,s Guys thanks
 

flattop1

Dawg 89
port matching highly unlikely
Carb to big maybe
rockers again unlikely
You did not try the needle?
so up till1/2 throttle all is good when you slamit down it starts sputtering?
or full throttle till it reaches say 4000rpm then sputters?
 
port matching highly unlikely
Carb to big maybe
rockers again unlikely
You did not try the needle?
so up till1/2 throttle all is good when you slamit down it starts sputtering?
or full throttle till it reaches say 4000rpm then sputters?

yes I tried the needle in 3 different spots
and it go,s way past 4000rpms
Iam thinking bigger jets a friend said when I was running hot laps that my header was glowing a little up close to the head
 

Kart43

Member
It is lean up top you need to get the mixture at wide open richer. I don't think you are getting to a point where the valves are floating so rule springs out sorry.
 

zenawarrior

New member
It is lean up top you need to get the mixture at wide open richer. I don't think you are getting to a point where the valves are floating so rule springs out sorry.
For a few years I've been chasing "splutter or pops through carb" changing mixture, carbys, coils, pumps, fueltank, just about all the guys at the track tried to help, and some saying it's not valve bounce. But it was, stronger springs, and it all went away. If you got doubt take your manifold and carby of and look inside from valveside and if it's black, it's sod from burnt fuel blowing back out through the carby. It will have a clean path in the carby where fuel comes out. You can rev the engine too with the filter of and it should blow fuel out, more than normal, relativ to the type of cam. Please let me know what you find, so I can rewrite my "book"
 

Kart43

Member
He says it starts sputtering at 3/4 throttle, you could be right but there is not enough information given here to start changing parts. He has a 34mm motorcycle carb on this thing if it is not tuned to the engine he can switch all the parts he wants. He said he had dual springs installed did not state what pressure but I will guess, as he has nr rocker arms perhaps he has their dual springs at 65lbs. What weight springs will you suggest, you have as much information as I do. My book can be edited, it does not need rewritten.
 

zenawarrior

New member
He says it starts sputtering at 3/4 throttle, you could be right but there is not enough information given here to start changing parts. He has a 34mm motorcycle carb on this thing if it is not tuned to the engine he can switch all the parts he wants. He said he had dual springs installed did not state what pressure but I will guess, as he has nr rocker arms perhaps he has their dual springs at 65lbs. What weight springs will you suggest, you have as much information as I do. My book can be edited, it does not need rewritten.
Good one mate, yeah I guess mine can be edited too, actually it's writen with a pencil. I used to have GP960 but at about 8900 for one engine and 9200 for another, it would have a "missfire" that could be heard by others. The difference between the two was one had 1/4" stem valves , the other 5.5mm. Now I run GP 965 and rev to under 11000, to much I know. If I could just find a cam with the same power, at say 8500 would be great.
 
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