only runs with choke on

fullercameron

fullercameron

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Have been reading through multiple posts about this topic, but haven't been able to solve the problem yet. If this is covered somewhere and I didn't see it please let me know.

I've got the bike running and it sounds reasonably good. I have been through the carb guide multiple times and was hoping to begin syching and adjusting in earnest but I can't get the bike to run with the choke off.

It starts on the first or second kick usually but after a couple of minutes at idle, when I reach down to switch off the choke the bike immediately dies. It is idling a bit high (around 1500) throttle was causing a bit of popping on engine deceleration but after double checking timing, valves, point gaps, cam chain adjustment and plug gaps, throttle seems to be responsive and returns to idle quickly after a blip on the handle - only with choke on still.

I've got the carbs off again and I am re-cleaning and checking float level, etc. The PO had upgraded to a 750 kit years ago, but in going through things, I ended up replacing the cylinders and going back to the original 650. Checked to see if he had re-jetted the carbs, but they still have the 130 main and 42.5 pilot that came stock with the carbs from 72.

I am running aftermarket turnout mufflers (that came with the bike) and pod filters from mike's XS. I was hoping someone might be able to give me some advice that would allow me to run the bike without the choke so that I can see about getting things straightened out. Plugs are reading rich (carbon black), but this seems right since I am unable to switch off choke.

Since this problem is at idle, I assume that it is a pilot circuit problem but that is as far as I am getting. My guess is that when the choke is switched off, it leans the mixture to the point that it can't run. Does this sound right?

I ordered a size larger pilot jets to test, but would like to see if someone has some advice before I start really tweaking things.

I'd appreciate any help. Still in the process of learning to be someone future owners don't curse for my mistakes and this forum has been a great help.

Thanks
 
fullercameron;

Since you need to have the choke on, that tells you that your pilot circuit is almost totally blocked off or your float levels are set to low.

Are you sure the float levels are correct?

The pilot circuit must be confirmed clear. There are 3 areas to look at............the mixture screw, the pilot jet, and the small exit holes in the carb throat. You can remove the mixture screw and spray carb cleaner (use safety glasses) and compressed air until you see it (feel it for air)come out the bypass holes and the single hole in the carb throat,( cover the pilot jet to force it out the small holes.) Cover the holes and blow back through the pilot jet.

Have you tried adjusting the mixture screw? 3/4 turn for starting for 72 year.

You should also confirm you can blow air through from the pilot air jet.
 

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Retiredgentleman

I will double check the entirety of the pilot circuit and recheck the float levels. I have the mixture screw set at the 3/4 turn starting point and have turned it in and backed it out a couple of times.

I have inline fuel filters and cleaned the tank with evaporust but did notice some rust colored fine sediment in the float bowl that could possibly be gumming up the pilot circuit.

Going out of town for a few days to visit family but will give a status update when I return.

Glad to know that chasing problems in the pilot circuit is at least in the right general direction.

-As for the idle screws, I have only turned them up and down slightly to get the exhaust sound relatively even and get the idle speed close (like I said it is still running a bit fast) but I wanted to get it running without the choke before I tried to dial it in to the proper speed.

Thanks for the help.
 
On my 75 with both the BS38 and BS34 carbs with the choke on at start up the idle speed climbs up to around 3000 rpms. Turn off the choke and the idle drops to about 800 rpms. I turn up the idle screws to get a 1200-1500 idle then as it warms up I back off the screws to keep the 1200-1500 rpm idle.
On your 72 messing with the idle like that can mess up the carb sync. My carbs are both linked and only have one idle speed screw for both carbs.
I would start by turning both idle speed screws in about 3/4 turns, this should bring up the idle speed so you can run without the choke.
If not try another 1/2 turn.
Once you get it to idle with out the choke and warmed up a bit then you can sync the carbs and set the idle speed right. Then after that just keep the throttle open by hand until it warms up so it runs without the choke.
Leo
 
^ I think that's a good suggestion. Here's another way of approaching it the same way as that.

As the bike warms up with the choke out, tap the choke in a little bit every 30 seconds or a minute to keep the speed around 1200 rpm. If you can't tap it all the way in and have it idle at 1200 after a few minutes, turn the idle screw up until you can. A sign that I have the choke out too far for a certain engine temp is the revs will hang high if you blip the throttle. Tap the choke in a little until that doesn't happen but you still idle at 1200 rpm, to find the right choke position for that temp.
 
xjmx the earlier carbs (pre 80) didn't have multiple choke settings. It is pretty much on or off. There may be some of the 78-79 Specials that came with 34's that would be the exception. All the 38's I've had single notch chokes.
 
^I've never heard of any choke on anything before that was either on or off with nothing in between. Maybe there's just no notch there but is still a continuous choke...?
 
The 38 chokes are on or off, no in between unless you physically hold the lever (which is near impossible). As mentioned, I think the idle speed is just set too low.
 
^Never heard of any other choke in the world that wasn't continuous, including lawn trimmers. I think that's a big vote for the BS34.
 
All right. Got back from visiting family and cleaned and checked the carbs. Does appear that there was some crud in the pilot circuit of one of the carbs. Got it all back together and started right up. Was able to get it running for the first time with the choke off. Had to hold the throttle open a bit, but it didn't die immediately as it did before.

I shut the engine down because I realized that I had left the tach cable disconnected (I was also doing some work on the clutch because the plates were stuck together from sitting for too long) and I wanted to see where the idle was at so I could adjust it. Tried to start again and nothing.

Checked for fuel and it was good. Checked for spark and saw nothing. Pulled the tank and found that one of the coils was really warm.

I am trying to figure out if someone can help me with the proper way to test the stock coils. I pulled both units and tested with an ohm meter between the two wires that go to the harness. Both read at 4.5 ohms (my meter reads .5 when I test between leads so I am assuming that this is actually 4 ohms). I then tested between the spark plug lead on the coil and each of the wires and one coil reads infinity for both wires, but the other coil reads 11.8 on the 20k setting for both of the wires.

I believe that this indicates that one of the coils is bad but I am not completely sure how to properly test them. It looks like most suppliers are out of stock replacement coils until september and if they are fried, my inclination is to go to a pamco setup but I would like to isolate the problem before I start heading in an unnecessary direction.

Thanks again for all the help. I greatly appreciate all that this forum has to offer. Time after time I have found myself stuck and so far with the help of everyone here, I have managed to learn something.
 
You didn't test the coils quite right but I think it was good enough. One could be bad and the other on it's way out. Testing procedures are in the factory shop manual which is on-line for free here .....

http://www.biker.net/
 
5twins

Looked at the manual on this site and it looks like it is for later model bikes (78-81). I checked out how they test the coils for resistance and it looks like I did essentially the same thing, I just tested from the metal probe on the inside of the coil tower (plug wire disconnected) rather than the plug wire.

The manual lists 3.9 ohms +/- 10% for primary resistance and I got 4 which seems within range, but the secondary numbers I get seems definitely off. They list 8k +/- 20%,but I am getting on the one coil about 11.8k (about +40% or so) and infinite resistance on the other coil.

Just to double check, in your opinion is the coil with infinite resistance in the secondary bad, and the one with the higher than proper resistance on it's way out?

Just want to make sure I understand what I am doing. Also, the manual lists testing method for points bikes, so do you think that this is the proper resistance values for the coils on my bike (72) even though the manual is for 78-81?

Thanks
 
Yes, you have the same coils as the newer points bikes and yes, the one with infinite resistance is probably bad, the other on it's way to being bad. With everybody going Pamco, you should be able to find some good used originals easily enough. I have a few but save them to help local guys with problems like yours.
 
5twins

Thanks for the continued help. I'll post something in the classifieds for the coils.

My hope is to get the bike fixed as close to now as possible. Got it all together a couple of months ago and have been playing with one thing or another since. Just hoping to get out and enjoy it a bit before it turns back to winter.
 
fullercameron;

Here's a simple solution, if you want to run with points for now, and maybe later go to a Pamco. Just buy one #17-6822 dual output coil from Mikesxs. That coil will work well with either points or Pamco. Cost about $37.00.

When used with points, you simply connect both sets of points together in parallel, and disconnect 1 of the condensers.

Those stock original ignition coils had weak voltage when new.....................if you buy used coils they could have even weaker voltage.
 
Also leaving the key and engine stop switch in the on position with out the engine running leaves power flowing through one of your coils. This overheats the coil. Burning it out. They often swell up and crack if left long enough. Thats the coil you found very warm.
I ran a dual output coil the way RG suggested for a short time. It works well. I tried the single points conversion and then put on the Pamco and are very happy with it.
Leo
 
This sounds like a good option. I did actually buy a set of used coils for cheap and was hoping that I could solve the problem by the weekend. If they work I'll likely keep these until the winter. After sitting on this bike for a couple of years I just want it to move with me on the back of it for a little bit.

If I have the cash, I'll hopefully get to the conversion when the weather turns bad. I have a bit of a romantic streak about using points - I like the fact that I can see how pretty much everything on this bike works. But in the end, although points aren't terribly hard to set, I'm not yet at the point that it doesn't take some patience on my part to get things dialed in. The pamco system seems to work well for everybody and looks easy and reliable - it's likely only a matter of time.

Thanks for all the input. I'll let you know if I end up wiring the points in parallel and need another round. One way or another this thing will be moving me(rather than the other way around) before the summer is out.
 

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