56 replies to this topic

[80sMiniCity]

Here’s the problem, I can rev the engine with no load without any issues. The problem is, the engine bogs down, to the point of stalling, when accelerating from a stop or basically in any gear under 3000 rpm. It looks like the piston is rising to quickly and bogging down the engine, but only under load. I put 2 red springs in and 20w50 oil, it helped a little but it’s really not drivable. I did the following with no change in driving.

Checked the timing
Checked valve clearance
Rebuilt carb, changed from waxstat to fixed jet
Set mixture with a colortune
Checked for any air/vacuum leaks, none found
Checked the choke and accelerator linkage

It’s a stage 1 998 A+ with an aaa needle.

HELP I am out of things to check…

[rally515]

hows the clutch operating ? when you said it bogs down , its worth a look to see if all is operating as it should

Edited by rally515, 22 December 2012 - 07:11 AM.

[absx2]

_{Over fueling will give the same symtoms of bogging down at low revs, maybe try some light oil in the dash pot. Are you using a cone filter or air box?. Also make sure the main jet is below the bridge section in the the carb as it will run like a pig if its too high due to incorrect atomization . Is the neddle jet set so the shoulder is inside the recess in the piston. Sounds stupid but it happened to me once, are the holes in the carb piston blocked?.}

Edited by absx2, 22 December 2012 - 09:16 AM.

[Skortchio]

First thoughts would be, when did the problem manifest and what did you do immediately before it started? Occam's razor and all that.

Is running 2 springs in your dashpot a common approach? I'd have thought they just get in each others way.

How are you viewing the pistons rise while driving, out of curiosity?

If you've eliminated all carb related possibilities, then you have to look at the other elements; ignition and fuel supply. If the carb works within normal tollerences, has no vac leaks on itself or way into the head then logic would say it's not at fault and the error is somewhere else.

The obvious things to confirm good would be plugs, coil, leads, (points if you have 'em), all the usual culprits.

[BritishRacingGreen]

Was it rolling roaded after you fitted the stage 1 kit? If not then it might be an idea to get that done.

[ANON]

hs4?

[80sMiniCity]

Sorry for the delay in responding, I’m in America.

It’s a stock HS4 with bigger manifold, LCB exhaust and aaa needle, a basic stage one kit. I can’t say when it started since I took it out of storage this past summer. I rebuilt the HS4 and converted it from a waxstat to a fixed jet. When I put the carb back on I had the issue. I could see the piston while driving since I had the center instrument out and the K&N filter off.

I originally had a single red spring and 3-in-1 oil in the dashpot. It rev’ed great just sitting but would bog down and stall under load.

I put two springs, not in series but sandwiched together, to increase the resistance on the piston along with 20w50 oil. That worked okay during the summer, but now that is in the 30’s here it’s starting to bog down again.

I put new plugs and leads on it this summer. Also tried changing out the coil too. It’s got a Metro electronic dizzy on it now, but the problem was there with the original 59D. I checked the plugs and they look good.

What would cause the piston to rise to quickly only under load?

[ANON]

any play in the spindle/body on the cable side?

[80sMiniCity]

No play in the accelerator spring, I actually replaced it this summer when I rebuilt the carb this summer. When I rev the engine sitting and idling there are no flat spots and it’s very responsive. This is the case with one or two springs and any weight oil in the dashpot. The bogging only happens with a load. It’s like if you raise the piston quickly with your finger when idling, it stalls.

[ANON]

not the spring, the spindle/body on the spring end.

[80sMiniCity]

Sorry, misunderstood. There is no play or air leaks on either side of the spindle. The spindle rotates fine with no kinking either. Something is causing the piston to raise too quickly under load. It has a spring needle that is set flush to the bottom of the piston.

[dklawson]

You have touched on many areas and I understand your frustration. I am unclear about one thing you said. You describe the situation as stalling/stumbling when accelerating BELOW 3000 RPM. Does the engine regain power and pull again above 3000 RPM or are you saying that it will not pull once under load and not once you reach 3000 RPM?

You have looked at so many fuel and ignition items that it may be time to look in other areas. Start with a leak-down test (not just a compression test). Determine that the engine is truly mechanically healthy. Also, look for and replace any fuel filters a previous owner may have installed. Confirm that the pump is delivering sufficient fuel to support your engine's demand for fuel and of course, check the HS4's float level if you have not done so.

[carbon]

Looks like you have covered most of the obvious stuff.

Just one suggestion to make about non-obvious problems. I had a problem with HS4 carb motor which started similar to what you describe, would not pull cleanly above 3000rpm but would sit perfect at idle and at lower speeds. For various reasons this was put away in garage for long time, as I did not have time to sort it out.

When I pulled it out of the garage 10 years later it would not start. To cut a long story short the problem was with the HS4 main jet assembly, to be precise the rubber gasket sealing the nylon hose into the base of the float chamber. This had perished and swelled to the point it had expanded and pushed into the fuel pipe with effect that no fuel was getting to the main jet, despite float chamber being full. Replacing this rubber seal (and digging the rubber grot out of the float chamber base) solved the problem. Now running better than ever before, so this bloackage was probably gradually getting worse over quite some time.

Suggest you check fuel flow through main jet right up to the 0.090 outlet is enough for your motor.

[80sMiniCity]

How do you check the fuel flow through the jet? I put a new jet and rubber washer on the jet tube where it connects to the float chamber. One thing I noticed was there was no metal tab on the float to adjust the level. Am I missing something or is the float not adjustable?

I used the colortune to set the initial mixture. I can’t use it to test under load.

[ANON]

the all plastic float isn't adjustable although assuming you need to close the gap you can use shim washers under the needle seat, you're looking for 0.062"-0.187.

did you have a washer between the nut on the jet tube and the rubber washer?

i'd be looking for a fueling issue, but take one of the springs out and put some light oil back in.

also worth giving the damper a clean and a light polish on the brass barrel and make sure it is free when you slide it into the piston before you build it back up.