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Discussion Starter · #1 ·
I recently purchased a junior Sprint car with a 1984 Honda 200m motor. We rebuilt the car from frame-up. Right now we are having motor issues where the motor seems like it has a rev limiter at around 3,800 to 3,900 RPM. I know the motor does not have a rev limiter. I am not a novice when it comes to Motors. I have been working on them for 20-plus years, but this has me baffled. the motor has a 12 to 1 compression piston in it I am running VP Race Fuel it has a oversize cam and a 34mm carburetor. I bought the car in pieces and when I first put it together so we could test drive it it seemed to run fine other than the idle was weird it seemed lean and only wanted the idle high. so I took the carb off soaked it cleaned it put it back together and it did the same thing. I purchased another carburetor brand new put it on and the motor ran the same way so I assumed it's a jetting issue. the carburetor that came with it had a size 40 pilot jet and a size 96 main jet the needle is set in the center position.the air screw on both carburetors did not seem to make a very big difference when I adjusted it either way. I tried putting a larger pilot jet in the original carburetor and it seem to affect the top-end more than it did the bottom end. it made the high rpms worse. I tried running a smaller pilot jet and it was worse then when I had the 40 in it. I've tried running a larger and smaller main jet and the original Main Jet seems to be the best. I'm getting plenty of fuel to the carburetor. I'm getting plenty of spark. I have tried adjusting the needle up and down but the center position seems to be the best. And I'm out of ideas. any help on this would be great.
 

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Nice looking cart Haw , what method are you using to checking the rpm, 3,800 sounds like 1/2 of what it should do
 

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Hi,

The air filter you have on it is way too restrictive. It will run a lot better if you can fab up a decent sized airbox (dead air space), put a large UNI foam filter inside, then snorkel the intake air into that box with a length of pipe or tubing. I'd start with about 1.75" diameter snorkel tube (smooth walled tubing) between 12-16 inches in length or so... until you get the carb & ignition dialed in for the motor & gearing etc. Then you can fine tune the induction for the powerband sweet spot & gearing by simply altering tube length & testing various drilled restricters at the entrance to the snorkel.

Are you running a stock Honda Ignition? In case it is a Honda supplied CDI on your motor... those modules ARE rev limited. You can buy a Ricks Electric No-limit CDI and solve that issue, or run something else...?

You will definitely have to rework the carb to get the jetting set up for the mixtures the motor needs. A Mikuni flat-slide carb model (may find a good used 34-36mm one on a dirt bike?) is probably going to work the best for you. Fuel pump pressure is pretty critical too... gotta run an adjustable pressure regulator & a gauge so you can lock it down precisely.

Have fun with it!
 

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Discussion Starter · #6 ·
The air filter on there is very free flowing. It runs the same with it off. It also runs the same with or without being under load. And i have a aftermarket tack that has a wire that wraps around the plug wire.
 

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Discussion Starter · #7 ·
Also it has a stock COD as far as I can tell. Specs I found on it says it makes max HP at 7200 rpm I believe. And it has an electric fuel pump with a 1-5psi regulator with a return line in the system. Its getting plenty of fuel and not to much.
 

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I have one of those tacks , remember when I first got it , it had to be reset it as it was set for a 2 cylinder and showing 1/2 of the actual rpm's -------
 

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I see a couple problems just by looking at your photo.

1) The lack of a proper induction/air cleaner/duct assembly as I mentioned above. The induction and exhaust systems must work as (and remain throughout the power producing RPM range) a symbiotic pair... Gross intake & exhaust port flow capabilities, valve timing vs cylinder pressures at/near max volumetric efficiency are some of the major determining factors (there are others to be taken into account as well) for intake & exhaust ducting requirements.

2) The header pipe is a very small diameter compared to its length. It is WAY too long... flow is severely restricted as is. If you need the header pipe to be that long you'll need to step it at least two times along its length before dumping into your muffler expansion. Its overall length vs gross flow capability should be optimized for the expected powerband (valve timing etc.) and the exhaust should flow at least 70% of the gross intake flow figure. These are very rough outlines...

You didn't mention what you are using for a carb...?
What are your camshaft specs...?
 

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Discussion Starter · #12 ·
I'm uncertain on the cam specs. The motor was built like this when i bought it. The intake valve was seized open in the valve guide when i got it. It came with a parts motor so I swapped heads. After I got the motor back together I put it in the car and we did some test laps. Other than a high idleing issue the motor ran well. At this time idleing was our only problem. These test runs had the same air cleaner and exhaust as it has now. I pulled the car apart to bare frame blasted, painted, and put the car back together. While I had the car apart, I cleaned the carb. Idle did not improve. So I figured the air screw air passage was clogged. So I bought the same size carb and put that on. It ran the same way as the original carb did. So I switched back to the original as the jets were different style in the new carb. (China). After this is when we noticed we had a top rpm issue that we didn't have before. So topend rpms were fine with the intake and exhaust it had at the moment. Something else changed. I know the exhaust isn't anywhere close to perfect. But there is something else wrong.
 

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Discussion Starter · #13 ·
Cam shaft lobes were considerably taller/more aggressive lift than the one in the stock parts motor.
 

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@fishfiles may have the answer. What does your spark plug reading indicate? Rich, lean... hot... cold... or OK looking plug?

What is the brand and model of the carb you are running on it now?
 

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Discussion Starter · #16 ·
Timing chain is all good. I can't remember off hand on the carb. Brand Looks like a keihnin knock off. Brand new plug with 5 mins run time was light brown with a little black on exhaust lalve side.
 

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You'll need to get a Japanese carb to fix it then, either a Keihin or Mikuni.

If I knew all the particulars of your motor mods... the vehicle weight (with driver) & gearing, track length & corner configuration (the track you'll race the most), the RPM range (from lowest to highest between corner shutdowns), I could help with a recommendation. The easiest carb to dial in will probably be a vacuum-slide Kiehin model... those are very lazy and forgiving. If your combo indicates it could benefit from a flat-slide Mikuni I wouldn't hesitate to take advantage... But the induction & exhaust should be optimized before attempting to dial in a good Mikuni.

Size the choice of carb (in CFMs) to your max RPM (+ 5%) and it should be easy to tune for any summertime racing conditions.
 
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