Keihin v Mikuni

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wls

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I have a 78 exciter 440, with I believe is the Keihin 44-38 butterfly carb, I also have on hand a mikuni vm 36 powerjet carb from a 77 440 yamaha powerd sno-jet. How similar in performance wise are these, I have never been around a keihin, but have tuned mikuni's before. What carb would give me better performance?? I'm setting this sled up for oval racing. Thanks
 
Well I dont think just putting on a slide carb is gonna boost your power way up, but from a tuning stand point the slide carb is alot more user friendly and has the ability to be tuned in the pilot, needle and main jet circuits where as the butterfly has no mid range tuning adjustments. the buuterfly carbs need to have passages drilled out in it to adjust and if you need to go back the other direction after you drilled it out, your stuck.
 
mrviper700 said:
Well I dont think just putting on a slide carb is gonna boost your power way up, but from a tuning stand point the slide carb is alot more user friendly and has the ability to be tuned in the pilot, needle and main jet circuits where as the butterfly has no mid range tuning adjustments. the buuterfly carbs need to have passages drilled out in it to adjust and if you need to go back the other direction after you drilled it out, your stuck.
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Ya my main concern was not to get less with the mikuni, I don't know how the flow simularites are between the two, they should be close, both are 440 yamaha's. thanks
 
You will have better tuneability with the round-slide VM as the emulsion tube (needle jet) is changeable along with several other parts. It’s probably got a 1.5 or 1.8 inlet valve so that’s good as well. You also have a ton of low cost parts to choose from. If the tube (needle jet) is fixed – I would stay with the Keihin unless you really understand needles (jet needle).

The stock Keihin is good as well. If you need to make some changes, you will have to drill out the passages – thread it to something like 4-40 and use air jets or whatever (even a set screw drilled out will do) from the present carb to adjust the mixture. You can make it better – but it’s more work than the old VM.

Also - the 77 fuel flow would be no where near the settings on the Mikuni as pipe and engine are very different from your model.

What you need to look for is better mid range using the VM, right off the stops – I doubt you will get more top HP.
 
nice read Bob. Hey, I got an Idea..... how about you take a trip out here to central minnesota and help me wrench and tune my SR292 and VMax540 oval sleds this winter!!!!!
 
The 76-77 Exciters used a BD44X38 Keihin slide carb on them and they worked well but the butterfly carbs used on the 78-81 Exciters were just as good if not better on a stock sled. I don't think 1 36mm Mikuni is going to do you any good and it will probably slow you down. You will also have to ditch your oil injection or switch to the 76-77 oil pump cable and thumb control. Those old Exciters will really go for a 440 fan and there is alot more left in them from stock but I wouldn't start with the carb. I know a guy that runs one with a modified Phazer top end but still uses the stock butterfly carb and pipe. It could use more carb on the top end but will stay right with SRX's and GPX's for the first 200ft. Freeing up the chassie, clutching, reeds, and some porting will really wake up an allready quick machine even with the stock carb.

opsled
 
opsled said:
The 76-77 Exciters used a BD44X38 Keihin slide carb on them and they worked well but the butterfly carbs used on the 78-81 Exciters were just as good if not better on a stock sled. I don't think 1 36mm Mikuni is going to do you any good and it will probably slow you down. You will also have to ditch your oil injection or switch to the 76-77 oil pump cable and thumb control. Those old Exciters will really go for a 440 fan and there is alot more left in them from stock but I wouldn't start with the carb. I know a guy that runs one with a modified Phazer top end but still uses the stock butterfly carb and pipe. It could use more carb on the top end but will stay right with SRX's and GPX's for the first 200ft. Freeing up the chassie, clutching, reeds, and some porting will really wake up an allready quick machine even with the stock carb.

opsled
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Thanks guys for the info, I will run the stock carb, I am putting a 102 on it. Hopefully I can get a service manuel for it, for offset, what hole means what in the secondary etc.
Any one have a good clutch setup for this sled?
 
I can get you a copy made of the Yamaha service manual for the Exciters. It covers ALL years 76-81, both 340 and 440 versions. It is the base 76 manual with all the suppliments for all the consecutive years with wiring and routing diagrams and specs for every year. It is comb bound with laminated covers and has the factory spark plug color chart inside. I sell them for $35 per with shipping. If you want one let me know.

Opsled
 
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