Tec H50 with Oval Bore Mikuni problem ... pics and Videos ...

Sixpac440

Sixpac440

#1
#1
I have a NOS Tecumseh H50 motorsports engine and I put a Mikuni 22mm oval bore on it.
It gets to 6500 rpm and stutters .. if the throttle is cracked wide open it bogs down.

I have played with the stock jet by drilling it open and adjusting the clip on the pin.
I have tried it with and without exhaust, no change.
I just got a new UNI air filter and tried it with and without it today with no change.

So it seems the exhaust and the air filter are not restricting airflow.
I made the intake myself, its the same size as the carb and moror, 1". I don't think there is a vacuum leak in it.

This pic shows the tank above the float bowl and the gas filter below the tank.


The gas line runs behind the blower housing next to the block like HS50's. Then it goes to a 1/4" 2-way brass barb that I took the barbs off one end to fit the smaller gas line the fits the mikuni carb.


This pic shows a spring I put around the gas line to keep it from kinking.


These videos were taken with brand new genuine Mikuni jets, a 125 main and a 17.5 pilot.
The UNI filter was on and the stock VT-8 Nelson exhaust hooked up.
I have a new 130 main and 20 pilot that are still new in the package.

The camera is aimed on the Tiny Tach.

This first video is the carb being opened slowly until the starts stuttering then backing off.
[video=youtube;usMJc3oOrcs]https://www.youtube.com/watch?v=usMJc3oOrcs&feature=youtu.be[/video]

The 2nd video is getting the RPMs up quickly then wide open throttle .. it stumbles and bogs, then I feather it a bit, the RMPs come up the WOT again ...
[video=youtube;kkLD5zUukgU]https://www.youtube.com/watch?v=kkLD5zUukgU&feature=youtu.be[/video]

No changes were made between runs.
 
Last edited:
Phil1958

Phil1958

#4
#4
looks like the fuel line is pretty high compared to the tank. the lowest level
where fuel will flow out of the tank is highest spot in the gas line. not much
pressure to work with when things are that close. carplaylb is spot on.
 
Sixpac440

Sixpac440

#5
#5
The H50 came with a new Tillotson and fuel pump ... designed to work together.
Will that fuel pump work?
Does the pump go in after the fuel filter?

Kevin
 
FirePowerMinis

FirePowerMinis

Active Member
#6
#6
Drilled the jet? To what size? 130 is more in line with what I'd think that wants. Try slowly applying the choke, a bit at a time, to see if more fuel helps?

And, the basics, valve lash, cam timing, ignition timing are solid?
 
Sixpac440

Sixpac440

#7
#7
FirePowerMinis said:
Drilled the jet? To what size? 130 is more in line with what I'd think that wants. Try slowly applying the choke, a bit at a time, to see if more fuel helps?

And, the basics, valve lash, cam timing, ignition timing are solid?
Click to expand...
The current main jet as a brand new genuine Mikuni 125 that has not been drilled.
Valve lash, cam timing and ignition timing are non adjustable from the factory. I have altered none of these.
 
CarPlayLB

CarPlayLB

Well-Known Member
#8
#8
Yes, you can use the pump that came with the engine. You will need to find a pulse point as well. Best place to pulse is from the manifold
 
T

the beekeeper

Active Member
#10
#10
Where ever you can install an intake vac port that won't interfere with anything else, you could also try the crankcase, but the intake pulse would be stronger.................Tom.
 
danford1

danford1

#11
#11
Sixpack440, I agree it is fuel related. The sounds floating valves makes is much different than what is going on it your videos. It sounds like it is just running out of gas at top end.
If you have a bog, that would be a separate problem as the float bowl should have sufficient gas at lower rpm.
Hmmmm, did you adjust your float level? If it is too low, it would run out of gas at top end.

I also agree you need a fuel pump. They are very easy to install. If you don't want to drill or tap your intake, there are other places you can hook up the pulse line.
Do you have two oil filler/checker plugs on your side cover? You can drill one and screw in a fitting for the pulse line. I had a Briggs Animal racing engine that had that set up. It worked great.
I think you can drill the intake anywhere. You can also drill and tap the carb near where it attaches to the intake. The fuel pump pulse
doesn't have any "flow" through it, just the vacuum pulses from the engine.
I have a brand new fuel pump I'm not using. I'll send it to you totally free if you want it. It is the rectangular shaped one not the round one pictures above. The shape doesn't matter. They work the same.
If you want it, PM me your address and I'll get it right out.

Danford1
 
ole4

ole4

#12
#12
I can not see your exhaust in the picture. How long is it? Does it pull strong up to 6500 then start stuttering? If so first make sure it is not a fuel problem, Some pumps can cause the chikuni to flood and may cause other issues but will at least confirm if it is a fuel starvation issue. If your pipe is the wrong length it could be a reversion issue. If you could run it up to the stutter point running without an air cleaner, see if there is a cloud of fuel mixture at the mouth of the carb. Also if that is the case the plugs should pull out black if you let it stutter for a while. The fact that it is stuttering and then you open the throttle and it stops stuttering and bogs still sounds like a lean condition, kill it when it bogs and see what the plug looks like. if it is white it is a lean condition. You will need a few plugs for testing cause if you start testing with a black plug it is not going to turn white in one run.
 
Sixpac440

Sixpac440

#13
#13
Thanks for the offer amigo! I have a pump that came with the Tec Racing motor ... I'm hoping there is a place in the crank case that was made for fuel pump pulse ... if not I have a drill bit and some JB Weld ... JUST KIDDING!!!

I did not alter the float level and was careful not to bump it when the bowl was off ... how is it checked?
I am familiar with setting float levels, just not in Mikunis.

The bog is a top end issue, not a low/mid issue ... you hear it WOT in the video


danford1 said:
Sixpack440, I agree it is fuel related. The sounds floating valves makes is much different than what is going on it your videos. It sounds like it is just running out of gas at top end.
If you have a bog, that would be a separate problem as the float bowl should have sufficient gas at lower rpm.
Hmmmm, did you adjust your float level? If it is too low, it would run out of gas at top end.

I also agree you need a fuel pump. They are very easy to install. If you don't want to drill or tap your intake, there are other places you can hook up the pulse line.
Do you have two oil filler/checker plugs on your side cover? You can drill one and screw in a fitting for the pulse line. I had a Briggs Animal racing engine that had that set up. It worked great.
I think you can drill the intake anywhere. You can also drill and tap the carb near where it attaches to the intake. The fuel pump pulse
doesn't have any "flow" through it, just the vacuum pulses from the engine.
I have a brand new fuel pump I'm not using. I'll send it to you totally free if you want it. It is the rectangular shaped one not the round one pictures above. The shape doesn't matter. They work the same.
If you want it, PM me your address and I'll get it right out.

Danford1
Click to expand...
 
Last edited:
Sixpac440

Sixpac440

#15
#15
No matter what I've done it has stuttered at about 6500 since it was first started. A little lower RPM with different settings. When the restrictive air filter that came as part of the "kit" was on it did it, no air filter it did it, the new UNI air filter, still does it, exhaust removed, jetting at 100, 119 & drilled too big to 150, doesn't matter, it wont run smooth above 6500.
The exhaust is a stock VT-8 Nelson Muffler. 8" long x 3" diameter.
The bog is specific to this combination ... it hasn't always been that pronounced. It only bogs on top at WOT right now.
When I'm getting the RPMs up .. it hits a point and starts stuttering ... I can back off a bit and it smooths out ... if I'm at the point of stuttering and crack the throttle open it will bog ...

Today I checked the plug after WOT and it was white.
Then I got it up to 5500/6000 where it runs smooth and checked the plug .. a little light but it looked good.

Kevin



ole4 said:
I can not see your exhaust in the picture. How long is it? Does it pull strong up to 6500 then start stuttering? If so first make sure it is not a fuel problem, Some pumps can cause the chikuni to flood and may cause other issues but will at least confirm if it is a fuel starvation issue. If your pipe is the wrong length it could be a reversion issue. If you could run it up to the stutter point running without an air cleaner, see if there is a cloud of fuel mixture at the mouth of the carb. Also if that is the case the plugs should pull out black if you let it stutter for a while. The fact that it is stuttering and then you open the throttle and it stops stuttering and bogs still sounds like a lean condition, kill it when it bogs and see what the plug looks like. if it is white it is a lean condition. You will need a few plugs for testing cause if you start testing with a black plug it is not going to turn white in one run.
Click to expand...
 
Studeman68

Studeman68

#19
#19
I'm a snowmobiler from the past, and I had some pretty tricked out stuff... with Mikunis on them..What you have done with that carb seems to be spot on... I'd say you are starving for fuel... when you feather it you are giving it a chance to catch up some....
 
Sixpac440

Sixpac440

#20
#20
Studeman68 said:
I'm a snowmobiler from the past, and I had some pretty tricked out stuff... with Mikunis on them..What you have done with that carb seems to be spot on... I'd say you are starving for fuel... when you feather it you are giving it a chance to catch up some....
Click to expand...
From what Ive learned about my Mikuni in the past 2 weeks I would say the air filter is good now, the jetting is a tick light and I will bump it up but its good enough to sort out the stumbling and maybe the bog ...
First up is get the fuel pump going ...

Kevin
 
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