4mil stroker problem learn on WOT even with NO HIGH JET

[Jr.]

From what I read, its not a carb or jetting issue. Its a supply issue! OP states he tried two sets of carbs with same issue? That leave the supply from tank to carbs as the target problem area? The issue as I see it is not enough supply when the need is greatest? Look at too small dia fuel lines that possibly collapse under suction?
Running too small inline fuel filters? Or filters with restricted flow such as bronze sintered element filters?
Stuck ck valves?
The basic remedy would be two dedicated feed lines to each carb, Min .250” ID. High capicity fuel filters on each line. Verifying unrestricted flow from tank pickup to each carb. And lastly, make sure the tank is properly vented.

Good lucK!

Ski Ya, Paul

 

[sambo]

Some specs of the engine and results

Engine
-62t ported cases with epoxy ramp intake.
-61x sleeves port matched.
-4mil adr crank shaft
-Ada head
-Bored to 84mm pistons
-Squish 42 thou
-175 psi compression.
-Carbon tech reeds on stock 62t cage
-daul pulse lines
-Engine hold pressure of 10psi and drops 1psi every 30mins.

Carbs
-New 46s stock undrilled return line.
-2.3 ns
-21psi pop off (have tried as low as 16 to 22 psi)
-doubled up on check valves on kidney plate
-1x mesh screen in a stock air box for dauls
- jets started off 120 low and 120 high as recommend. Low speed circuit was best as 135 low jet with 21psi i can get it to bog onnlow spped circuit with 3 turns and dial in crisp at 1 turn on low.
- high jet circuit will never bog even with 190 jet or completely removed and 3 turns out on screw and still pulls hard at WOT.

-fuel filter and fuel tap have both been bypassed with no change. Cha he's fuel lines to reforced that so will not collapse. Pulse lines are now hard oem lines.

Fuel pressure between carb and return jet was
- 60 return. 2.5/3psi idle , 8/9psi full throttle
-80 return jet 2.0/2.5 psi dle, 7/8 psi full throttle
-95 return jet 1.0/1.5 idle, 2.5/3psi cruising just off throttle and 5/6 psi on full throttle

I have stuck with the 95 return jet now.

Exhaust
- mod pipe camber
-bpipe bored to 50mm
- top open 1/3. Mid close. Bottom open 1/3 (water temp cold in the UK)
- 2.5mm stinger restrictor

 

[sambo]

Reading the message that Vortex sent about mikuni trouble shooting chart. Seems the the carbs could be too big for the engine. As I changed the jets up on high and no difference. So that leaves carb a too big or too restrictive. I will try 3 mesh screens as I'm only on ine screen but can't see that being the issue :/

 

[Storbeck]

I'll get some reading of today's test up later. But yes I was getting a lean hesitation when snapping open throttle from 1/4 throttle, falls on face then rips my arms off. Bit I thought a super large high speed jet would sort it or make splutter with fuel bit nothing changed when going larger jets.

Some things that need to be made clear:

1. If your problem is happening when you snap the throttle open, it's not in the high speed jet, it's the low speed or popoff. If you have something weird in your high speed circuit (huge jet, no jet, screw way out), put something reasonable back in, whatever was working well at high speed before, put that in and work on your low speed jet. Once your low speed is set you'll have to revisit the high speed, but super rich high speed circuit will not fix a lean hesitation when snapping open the throttle.

2. If you have the stock restrictors in the carb undrilled, you shouldn't be using a return jet, and your pressure readings are not valid. If you have them drilled, then that all looks fine.

3. What do you mean by this:

"- jets started off 120 low and 120 high as recommend. Low speed circuit was best as 135 low jet with 21psi i can get it to bog onnlow spped circuit with 3 turns and dial in crisp at 1 turn on low. "

If you can make it "Crisp" what is the problem? Was that result with the 120 high speed jet, or was that with the super huge jet or no jet?

 

[sambo]

Some things that need to be made clear:

1. If your problem is happening when you snap the throttle open, it's not in the high speed jet, it's the low speed or popoff. If you have something weird in your high speed circuit (huge jet, no jet, screw way out), put something reasonable back in, whatever was working well at high speed before, put that in and work on your low speed jet. Once your low speed is set you'll have to revisit the high speed, but super rich high speed circuit will not fix a lean hesitation when snapping open the throttle.

2. If you have the stock restrictors in the carb undrilled, you shouldn't be using a return jet, and your pressure readings are not valid. If you have them drilled, then that all looks fine.

3. What do you mean by this:

"- jets started off 120 low and 120 high as recommend. Low speed circuit was best as 135 low jet with 21psi i can get it to bog onnlow spped circuit with 3 turns and dial in crisp at 1 turn on low. "

If you can make it "Crisp" what is the problem? Was that result with the 120 high speed jet, or was that with the super huge jet or no jet?

The 120loe

"doubled up on check valves on kidney plate " I find this to be problematic as now it is harder for fuel to flow thru the High Speed Circuit, this is counter-productive.

The Check Valves that are normally doubled-up on are the Check Valves on the PUMP side, not the REGULATOR side (which houses the Jets).

I can try just 1 check valve. Although mikuni mention about running 2 and I have always run 2 in all my motors. But it's another thing to try. I'm all ears willing to try any ideas now lol

 

[Storbeck]

"The 120loe"

What?

 

[sambo]

I think that's a type0

 

[Storbeck]

I asked three questions:

1. What do you mean by that statement

2. If you can make it crisp what is the problem

3. Was that result with the 120 high speed jet, or was that with the super huge jet or no jet?

To which of those questions was that the answer, and what word were you trying to say?

 

[Big Kahuna]

Have you followed Jr's advice at all? What he said makes sense. Sounds like a fuel feeding issues. I have had fuel line suck air at the Primer "T" before, ok on a slow pull, but on high demand it would start sucking air.

 

[TheBuzzard]

The 120loe

I'm all ears willing to try any ideas now lol

i'm still waiting for you to but the bigger pilot jet in there and then get back on here any say the lean hesitation went away.

 

[sambo]

I think you are referencing page 23 of the Mikuni Manual, 3rd Paragraph where there is a Rich Condition after a High Speed Run, caused by excess fuel in the Carb, This does not appear to be your case.

The last words on this Mikuni paragraph also states "in some cases extra valves can adversely affect throttle response"

This makes sense. Hope this is the issue

 

[sambo]

I got a 145 pilot to try. Weather was to bad for testing today.

 

[SBrider]

how thick is that spacer plate, and what is the hight of that exhaust port, i had a similar issue with a 819 i built a few years ago, super lag on the bottom, kept fatting up the low speed, was helping but never fixed it, the motor was stupid fast on the topend, but huge dead spot around 1500-2500, then would come on like all hell, i believe it was due to exhaust port timing too high, when i removed the spacer and whent to a cut head " squish 2.5mm" , all my lowend came back and had to reduce my low speed jetting to get the crisp crackle!! back .. this was on blackjack 44s, 135 low 140 high 2.5s i think, not sure i could check back in my notes, left the timing stock but had a msd enhancer , anyways just what i have experienced, maybe different parameters in ur case, but high port timing will give u a lower end lag that would feel the same as a lean issue.

 

[Big Kahuna]

how thick is that spacer plate, and what is the hight of that exhaust port, i had a similar issue with a 819 i built a few years ago, super lag on the bottom, kept fatting up the low speed, was helping but never fixed it, the motor was stupid fast on the topend, but huge dead spot around 1500-2500, then would come on like all hell, i believe it was due to exhaust port timing too high, when i removed the spacer and whent to a cut head " squish 2.5mm" , all my lowend came back and had to reduce my low speed jetting to get the crisp crackle!! back .. this was on blackjack 44s, 135 low 140 high 2.5s i think, not sure i could check back in my notes, left the timing stock but had a msd enhancer , anyways just what i have experienced, maybe different parameters in ur case, but high port timing will give u a lower end lag that would feel the same as a lean issue.

That is a good possibility.

 

[sambo]

how thick is that spacer plate, and what is the hight of that exhaust port, i had a similar issue with a 819 i built a few years ago, super lag on the bottom, kept fatting up the low speed, was helping but never fixed it, the motor was stupid fast on the topend, but huge dead spot around 1500-2500, then would come on like all hell, i believe it was due to exhaust port timing too high, when i removed the spacer and whent to a cut head " squish 2.5mm" , all my lowend came back and had to reduce my low speed jetting to get the crisp crackle!! back .. this was on blackjack 44s, 135 low 140 high 2.5s i think, not sure i could check back in my notes, left the timing stock but had a msd enhancer , anyways just what i have experienced, maybe different parameters in ur case, but high port timing will give u a lower end lag that would feel the same as a lean issue.

Erm I never measured the plate it came with the 4mil kit from Matt holeshot at HPE. Not sure on the gaskets thickness I want to say I was on 16thou gasket and 30thou. He gave a few and was told to change them around to achieve the correct squish now I know this changes the timing ever so slightly and the correct way to do it is machine the domes but was told it wouldn't be dramatic. Wouldn't this still be a issue on 38s though too.

I havnt change the timing in the cylinder with porting. Just exhaust port slight widened at top I'll see if can find a pic

 

[CARBONX2]

how thick is that spacer plate, and what is the hight of that exhaust port, i had a similar issue with a 819 i built a few years ago, super lag on the bottom, kept fatting up the low speed, was helping but never fixed it, the motor was stupid fast on the topend, but huge dead spot around 1500-2500, then would come on like all hell, i believe it was due to exhaust port timing too high, when i removed the spacer and whent to a cut head " squish 2.5mm" , all my lowend came back and had to reduce my low speed jetting to get the crisp crackle!! back .. this was on blackjack 44s, 135 low 140 high 2.5s i think, not sure i could check back in my notes, left the timing stock but had a msd enhancer , anyways just what i have experienced, maybe different parameters in ur case, but high port timing will give u a lower end lag that would feel the same as a lean issue.

2.5mm is huge squish, the general rule is no more than 1% of rod length

 

[sambo]

I'm on 42 thousands of a inch. (1.06mm)

 

[SBrider]

true, what i was meaning to say was the domes need to be cut another 2mill or 2.5 to accommodate the extra stroke when removing the spacer, unless running shorter rods, i have had non stroked motors have similar issues do to over porting, mainly exhaust or possibly too much case volume compared to cylinder . just my 2c.

 

[Storbeck]

I don't think those hpe kits have high port timing. Also, if he's doing them the way I think he's doing them, setting squish with the base gasket is the correct way to do it, and is the only way to ensure that the port timing comes out the way it is supposed to if he is porting the cylinder without having the complete bottom end in hand.

 

[CARBONX2]

What intake manifold are you running?