Carb problem

[oldskool]

My rebuilt 8hp briggs seems to run ok, but having high speed problems. The engine revs but seems to flutter and it seems to me it needs more fuel as it sounds like it can give more power. I have rebuilt the carb and tried adjusting the idle mixture screws and high sped screws and it still runs a little off. My question I guess is this carb worth putting time and more money into it, or should I go with a different carb setup? I heard on this forum that a Mikuni is the way to go. Is this ok on my briggs? Will it fit? How do you attach my governor to it? Or do you not? Heres some additional info on what I did to the briggs. I milled the head, new chrome rings, new valves (lapped), ported intake&exhaust, port matched carb, stock cam, rod,springs and finally astraight pipe about 14inches with muffler. Sorry for all the questions, but this kart project is taken awhile and my boys and me are getting impatient.

 

[anderkart]

Usually the governor system has been completely removed by the time most guys get to the point of installing a performance carb.

Unless you've already done it, you might just need to enlarge your stock carbs main jet. With your engine mods I'd think your running way too lean now. The way you could see if this is your problem is reading your spark plug: http://www.google.com/search?source...lz=1T4DMUS_enUS221US222&q=reading+spark+plugs

You'll find very small number-bits sold at good hardware stores. Take your old main jet along and find the bit size that just barely fits through it, then buy the next few larger sized bits. It'll be a trial and error type thing but keep enlarging your main jet and making full speed 15 to 30 second passes while suddenly killing the engine to get meaningful plug readings. You can get info from your old plug but you might want to start out with a fresh plug to make these tests more accurate.

 

[oldskool]

How do you enlarge the jet? What I mean is, do you enlarge the part of the jet where the needle goes in? If so, don't i then need a new high speed needle? or is it enlarged at the other end were its inside the bowl area? Thanks for the help and i hope i explained it correctly.

 

[anderkart]

I'm not familiar with your model but I've seen some automotive carbs that used a needle through the main jet. (sometimes we use smaller needles instead of larger jets in those, but either way it would richen the air/fuel mixture) With or without a needle, your main jet should be the biggest one drawing fuel from the float bowl.

If you spark plug reads that you need to be richer, just make a series of small changes each time until you get it right. (if you get it too big, it'll be harder to go back leaner)

 

[oldskool]

Thanks anderkart, i tried it and it ran slightly better in the higher rpm but it still cut out and fluttered a little. So i guess i will try to open it up a little more to see if it improves. My model is from 1976 8 hp briggs with an updraft carb. If this doesn't work should i go with a bigger carb?

 

[anderkart]

I'd think your old carb could be fattened up enough to work pretty well here. When I rejetted my 10 hp Honda for an open pipe/race air cleaner and govenor removal I ended going up about 3 number bit sizes larger to get my plug reading from a chalky white to a nice tan color. You didnt mention if your work today showed any improvement in your plug reading but if it ran a little better it sounds like your heading in the right direction and need to go further.

I've tried researching performance 8hp Brigs motors before and didnt find much info on the net. I have no idea what size Mikuni would work best for yours. Maybe a 24mm, thing is even if you get the right size you still sometimes have to experiment with adjusting its needle up/down or trying diferent jet sizes to get them dialed in and jetted right for your motor too. Although they have more performance potental, you probably wouldnt get all a Mikuni's benefits with your govenor still limiting your rpm.

 

[oldskool]

My plug actually quit and i put another one in. What was weird is i just had the engine running to try to adjust the carb and i stopped and an hour later it wouldn't start. After several checks it turned out my NEW plug gave no spark. Now the engine is hard to start It takes alot of pullingit's still running rough and popping out the exhaust. I still think its running lean. it won't idle i just try to keep it running by playing with the throttle. I am getting frustrated, i think i'll look for a replacement carb, maybe off my old 6.5 hp techumsi or look in scrap yard. Does anybody have Any suggestions???

 

[skid20]

After several checks it turned out my NEW plug gave no spark. QUOTE]

did you check your coil?

 

[oldskool]

I checked for spark when i pulled my plug and there was no spark, so i took a plug from my mower and tried it and i got a good spark. i never really had a new plug go bad so fast, it was one of those pricy split forked E3 plug. Oh well, i think i have convinced my self to stop spending time and effort on the updraft carb and instead look for a mukini or something else. does anyone have a suggestion as what is the best carb for my 8hp or were to find it cheap like dirt cheap, i'm going broke on this project?

 

[anderkart]

I searched around a little more and found this link for ya: http://www.g-team.us/briggs_8hp.htm

 

[oldskool]

Thanks for the link Anderkart, pretty cool mods for 8 hp briggs! I will look into more when i get this carb issue figured out.
Well i may have a carb from a honda atv 200. Its a Keihin??? Anyone heard of it? Is it an off brand or is just junk? Thought I would ask for some opinions if this was a good carb or not and if its worth it.

 

[theo]

I put a 25mm mikuni on my 8hp briggs. The carb was off a 80cc suzuki 2 smoke so I had to plug up the oil injection tube. Also I went up on the main jet, but not sure on how much it was a few years ago.

 

[572bigblock]

Looks like a neat, compact little setup you have there. ^^^^^

Does anyone have a good resource for identifying mikuni carbs?

 

[theo]

Do you mean type of mikuni or the size?

 

[anderkart]

Hey oldskool, the guy at that link seems to sell intake manifolds and Walbro carbs sized/jetted for your motor. (these Walbro race carbs come on Briggs Animal race kart motors and look just like round slide Mikuni's)

Keihin's might not be the highest performance carbs in the world but they usualy work just great. They've came on tons of Honda bike models for years and years, heck my 79 Harley Superglide origanaly came with a Keihin too.

 

[solomon]

how exactly did u mount that carb, i have a carb just like that on my honda that died. I have a 5hp briggs how would i go about mounting that?

 

[theo]

I cut the updraft intake manifold up and used part of it then a piece of rubber hose. For the 5 hp you could use a 1" washer and drill 2 holes in it then weld on a piece of 1" tubing a inch or so long.

 

[solomon]

that sounds like a really good idea but id have to make an opening at the end of the 1" pipe that is like 1 3/4" to adapt the carb. Do u think it be way too much power? its currently ungoverned and im gonna guess running around 4300 rpm, how much do u think the carb would increase it?

 

[theo]

I would say that the carb is to big and that's not good. You should look for a carb that is around 24- 26mm, if you go to big on the carb the performance from the engine will drop.

 

[solomon]

wel i have a 31mm carb mikuni would that work?