1974 xlch;
open pipes, Autolite plugs H10C gapped at .40, Dyna S (checked for spark)

History: I picked up a new VM38-9 and had no trouble when first installed. Meaning the idle screw was turned in to just make contact with the slide valve and the air screw was turned out ¾ of a turn. 3 weeks later I ran a bit low on fuel on a 3.5 gal tank and started to get popping and back firing. I filled up and had trouble getting the bike to start. I opened the carb and cleared/cleaned the pilot jet and air ways.

I put the carb back together and no longer have popping and back firing however;

Upon start up I engage the enricher but the idle thumb screw MUST be screwed in almost all the way (the spring completely collapsed) for the bike to start. Occasionally I have to crack the throttle while kicking. I was under the impression that the slide valve should be completely seated when using the enricher for it to get the proper mix of fuel and air. Also, sometimes after having trouble I pull the plugs thinking they would be fuel soaked only to find them BONE dry, how can that be, I have fuel in the float bowl? I do also notice once started I need to almost keep the idle screw turned all the way in with the air screw turned out 1 1/2 turns to keep the bike charging the battery.


Thanks guys

 

First off, I would set the plug gap to .025 to.030. A gap of .040 is really a bit too wide. Reset them and before you do anything else crank it up and see if there is any difference.

The Mikuni should be bullet proof and without any problems.
I would venture to say that you running low on fuel and having it start popping and back firing was nothing more than coincidence.

You say after you filled the tank it was difficult to start.
This is because
(A) some crap got into the carb when you were running on the dregs of the tank, or..........
(B) It is another problem entirely.

Since you say you disassembled the carb and put it back together and the bike no longer popped and backfired, that makes me think that you may have put something together in the carb that is not quite correct.

The clip on the needle inside the slide should be on the middle notch. That is the third notch from the top or bottom.

The air jet should be adjusted the way it was in the beginning when you first installed the carb. And the idle screw adjustment should be just where it first was....... just raising the slide off it's seat.
That's it, except for the float adjustment there is no other way to adjust this carb. And as I said before, this carb will be one without issues. I have had a Mikuni for 38 years and had not one single issue with it.

So I suspect your problems are elsewhere. Start by setting the plugs correctly.
Then check the ignition points gap. The lobes on the ignition cam (front and rear cylinder) should both gap out at .018 and be within .004 difference or less between the two.

Do one thing at a time so you can determine what causes the problem.

With the engine stone cold, adjust the pushrods too. All of this stuff makes a great difference between the bike running, running poorly, and not running at all........... pg

 

I am not familiar with a Dyna S, but the electronic ignition that i am familiar with does call for a gap of 040; Crane Hi-4 for example.

Check the fuel filter. I would guess that it is clogged with crud from the tank. And if so have the tank cleaned and lined - use either POR or RedKote, not Kreem.

 

I don't think its your carb. I had same situation with my 74 XLH. New VM38, good for a few days and then everything went to hell. Also had to bury the idle screw to keep it running. Would not take any fuel under load. Had my exhaust pipes glowing red. Of course I assumed it was the Mikuni since that was the most recent change. With Pinion Gears help I addressed the ignition system. Found a broken spring on the advance unit. Fixed it and problem went away.
Most recently she started running rough again and again I thought carb. Turns out the new condenser from China was bad. Got about two weeks out of it. Put an old one in and problem solved.
Aside from an oil leak all my problems have been timing related.
While I'm here who can recommend a decent brand of ignition parts. No luck with Accel. Mallory?

John F.

 

I am sure I put the carb back together properly. I will say the tank is in what I would consider “poor condition” as whatever the tank was coated with is starting to come apart (however I can’t visual see anything floating around). There is currently no fuel filter.

I gapped the plugs at .040 being that the website for Dyna S says between .035 and .040

http://www.dynaonline.com/skins/downloads/Default.aspx

What should I set my spark plug gaps to?
The gap settings listed below can be used as a starting point to obtain satisfactory performance with our ignition system. Any benefit from running larger or smaller gaps will need to be determined by experimenting with your particular engine combination.
Single Fire - Single Plug: .035 to .040
Single Fire - Dual Plug: .022 to .025
Dual Fire - Single Plug: .035 to .040
Dual Fire - Dual Plug: .035 to .040 - When wired so each coil fires a plug in each cylinder.
There are no points to set as there is only a module.

I sometimes do get the feeling that with the enricher engaged I can not get a venturi affect through the carb to bring in the fuel/air mixture. But that maybe because I have the idle screw turned in to far? Or I am just way off base as I can hear a “slurping” sound.

Timing

I can see how timing can cause popping and back firing; however wouldn’t it continue even after I took the carb apart and reassembled? Also once the bike was started and idling wouldn’t it run poorly if the timing was off? I must also note that the plugs would foul up terribly when this first happened although now after getting the bike started it has seemed to lessen.

I will say that “sometimes” I will get ignition at the very top end of the stroke of a kick. Which normally results in her kicking back, I try to get top dead center before kicking through so I am not sure if that means anything at all.

Either case I am going to check the timing tomorrow.

Piniongear: I am curious how a pushrod adjustment affects ignition. I had to replace push rod covers and washers due to an oil leak a few weeks ago. I set the push rods up with your recommendations here in the forum. How can a misadjusted push rod cause the bike to not run at all? The valves won’t open and close properly is what I am assuming you’re going to say.

I hope this wasn’t too confusing to read, thanks again guys and I included a picture of my ignition module.

 

Nice looking Scoot Danny. Was wondering how everything turned out..if Ya got Her running right?
Gene

 

Pushrods can be miss adjusted...
Engine must be stone cold, wait over night before adjustment.
Remove spark plugs, put bike 4th gear, lift rear wheel off ground, open pushrod covers.
Turn engine from rear wheel (correct direction) , see when intake and exhaust pushrods switch ( other close other opens) in front cylinder then adjust rear cylinder pushrods.
I have adjusted pushrods so that they turn freely but I can't feel any lash.
Then same procedure with rear cylinder...

 

LOL, my Chineese condenser lasted about two weeks too.

 

Here is my detailed pushrod adjustment notes ...

Ironhead Push Rod Adjustment

Do this procedure with the engine stone cold. I usually do it first thing in the morning.

1. Remove the spark plugs. Remove A/C to make room to work.

2. Using a flat blade screwdriver pry the spring keeper off of each push rod; the bottom of the keeper pops out first, then the top. Alternately, use a 3/4" open end wrench to push down on the top of the spring retainer, then pry out the spring keeper with the screwdriver.

3. The lower push rod covers may be stuck tight against the bottom o-rings, but they can be loosened by pinching between thumb and finger [or carefully using a shop rag and a pliers] and twisting or rotating. Lift these up and secure each with a tall-S-shaped piece of 12 or 14 gauge household wire [not cable] [or a long piece of Plumbers solder such as 95/5 hooked over the rocker cover and under the push rod cover.

4. Jack up back end of the bike so that rear wheel is off the ground, shift to 2nd gear, rotate wheel until it clicks, repeat until you get to 4th gear. Now by rotating the rear wheel you are rotating the engine. Almost impossible with the plugs in; very difficult in lower gears.

5. To adjust a particular push rod, its valve must be fully closed [the valve spring is relaxed]. Valve position for a particular cylinder is determined by observing the corresponding push rod of the other cylinder. For example, when the rear cylinder intake valve tappet is raised [valve fully open, spring compressed], the front cylinder intake valve is fully closed, and the front intake push rod can be adjusted.

6. Check [and adjust if necessary] the push rods:

[a] using the rear wheel rotate the engine so that the rear cylinder exhaust push rod tappet is fully raised. The front cylinder push rod is adjusted correctly if it has no vertical or horizontal play, and you can just rotate the push rod between thumb and forefinger with no trace of binding or dragging.

[b] to adjust if needed: hold the adjusting screw in place with a wrench; with another wrench loosen the split locknut. Then turn the adjusting screw using fingers only until the pushrod's ball end is seated in the tappet with a "slight amount of play". Then tighten further as follows:

Option 1: Adjust on the tighter end of the range: There should be no vertical or horizontal play, and you should be just able to rotate the push rod between thumb and forefinger with no trace of binding or dragging.

Option 2: Adjust on the looser end of the range: Set the adjusting screw to finger tight so that you cannot turn the push rod between forefinger and thumb. Undo it a tiny bit so that it is just barely loose. Then loosen it 1 "side" for an exhaust or 1/2 "side" for an intake. "Side" refers to one side of the 6-sided adjusting nut.

EDIT: Experience here is that Option 2 is the better and most likely the correct choice.

[c] hold the adjusting screw in place with the one wrench; with the other wrench tighten the locknut.

[d] recheck the adjustment; it is common for the adjustment to go out as you do step [c], in which case [b] and [c] must be redone.

[e] repeat for the other push rods.

7. Use a flat blade screwdriver to push the upper push rod covers up, and seat them into the upper o-ring or cork washer.

8. Install the spring keepers, top end first, by prying them in under the bottom with a flat blade screwdriver.

 

Did you ever get squared away with the buried Idle Screw. I am still battling the same situation. I have to keep it just about buried to keep a decent idle. I know that is just not right. I have adjusted the cable, reset the float, tried factory settings, intake air leaks, and I just can't get it right.
Yesterday I noticed a variation in the idle when I take the bike up off the stand. When she gets upright idle changes. Had to grab and twist the throttle to keep it running and then screw with the idle again.
Anyone out there make any sense of this?
PG what do you think?

Bucaman