Author Topic: AGRHHH Still won't idle! (Read 3410 times)

punch455 · « **on:** July 15, 2009, 06:04:15 pm »

I'm pulling my hair out. I've had my '78 750F for a number of years now. It never ran GREAT, but it got me around on the very few days I decided to chug around, especially to school and back. It always had a slight miss, and I knew the carbs needed a cleaning. Well, finally had the guts to do a complete teardown and clean of the carbs this past spring. When I say complete, I mean COMPLETE -- removed slows/tubes/needles -- everything. I even rigged up a little system to ensure the accel pump was working an squirting fuel through the little brass posts into the venturi.

Well, now it starts up, the revs float around 3000 rpm, and then plummet, and it dies. I can goose the throttle to keep it alive, but it eventually dies. I did a quick spit test on the pipes, 1-4 not nearly as hot as 2-3. So, I swapped the Y/B leads to the coils and swapped the plug leads, effectively switching the coils. Same problem. 1-4 warm. So, it's not the coils/wires. Bike has new plugs too. I then swapped the condensers to rule them out. Same problem. 1-4 timing is good too!

What do I do now? The bike is bone stock as far as the powertrain goes. Thanks in advance forum!

1timduke · « **Reply #1 on:** July 15, 2009, 06:09:06 pm »

Sounds like you're in bad need of a vacuum sync. I bench synced for a long time until I got a set of vac gauges, it makes a BIG difference.

-Tim

punch455 · « **Reply #2 on:** July 15, 2009, 06:14:03 pm »

I actually have a Morgan Carbtune, but the thing won't hold an idle. I have it connected and the only time I see any of the gauges show vacuum is right after I blip the throttle. After that they decrease all the way to the bottom.

bistromath · « **Reply #3 on:** July 15, 2009, 06:21:45 pm »

Check for air leaks on the rubber carb intake boots.

1timduke · « **Reply #4 on:** July 15, 2009, 06:23:34 pm »

Quote from: punch455 on July 15, 2009, 06:14:03 pm

I actually have a Morgan Carbtune, but the thing won't hold an idle. I have it connected and the only time I see any of the gauges show vacuum is right after I blip the throttle. After that they decrease all the way to the bottom.

When they go all the way to the bottom, is the bike still at 3k or so?

-Tim

punch455 · « **Reply #5 on:** July 15, 2009, 06:29:34 pm »

Well, it only holds at 3k for a moment after it starts, then it dies. I don't recall seeing the gauges during that time, but I will check again.

punch455 · « **Reply #6 on:** July 15, 2009, 07:15:37 pm »

No, no vacuum reading on the carbtune while its revving at 3k. I did a bench synch after the carb rebuild, so I don't think they can be SO out of whack that it won't hold an idle. Isn't that the point of the bench sync?

Spanner 1 · « **Reply #7 on:** July 15, 2009, 09:16:04 pm »

Could the floats be installed upside-down in the bowls ?.....if so low fuel level will immerse main jet, but the slow jet ( which sits up higher ) may be sucking air !

eurban · « **Reply #8 on:** July 16, 2009, 07:13:19 am »

Punch- I know you worked hard on your carbs but the idle circuits on these carbs, can be very tricky to get clean. There are a number of threads discussing the pulling and cleaning of the pilots as well as the other checks with spray carb cleaner to determine that all the interconnected passages are clear. It seems fairly common that one cleaning attempt is not always enough. How thorough were you with your cleaning? Are you certain that you have the idle mixture screw (IMS) installed with their parts correctly? How many turns are they set at currently? Are you sure that you have the slides installed with the cutaways in the correct direction (cutaway towards air box; they can be put in either way)? Did you visually check your float level settings using clear tubing on the overflow nubs? All should be exactly the same and just at the carb to bowl seam? Are your carb rubbers the correct ones for the F models and are they sealing properly? Have you checked for air leaks? . . . .Another non carb thing to consider is the condition of the engine. I assume that you have performed proper valve clearance adjustmens? You might consider checking the compression on your engine. Absolute compression numbers are not as critical (most automotive gauges will read lower than "actual" due to the volume of the gauge itself) as the differences between cylinders. Test and see how 1 and 4 compare to 2 and 3. 77 and 78 f models typically wear out their top ends at a significantly faster rate than the Ks so you may have an engine that is not operating properly. Good luck!

punch455 · « **Reply #9 on:** July 16, 2009, 09:11:18 am »

Thanks for the ideas everyone. I'll try to answer them as well as I can.

Carbs were completely separated and disassembled. I knew how difficult the slow circuits were at getting clean, so I spent alot of time on them. I used throttle cleaner spray to make sure that the pathways for the slow jets were clear. I could see the spray shooting up into the venturi. The slides were installed correctly too, because I did perform a bench sync and it would have been pretty hard to do if the cut-away was facing the engine. Floats were installed correctly and set with a digital caliper. 14mm I think was the float height. Anyways, IMS were installed (o-ring, washer, then needle) and set at 1.75 turns out. The rubbers are original and still in really good shape. I will do a spray test tonight to see if there are any leaks, but it's a little tricky in that the thing won't hold an idle for any amount of time. I haven't done a compression test for over a year or so, but I remember all 4 cylinders being pretty consistent. I will do another this weekend. Valves were a cinch to do, no problems there. Thanks again for the pointers! There has to be something blatantly wrong if the thing can't even keep an idle for more than a split second.

eurban · « **Reply #10 on:** July 16, 2009, 09:15:14 am »

Quote from: punch455 on July 16, 2009, 09:11:18 am

Anyways, IMS were installed (o-ring, washer, then needle) and set at 1.75 turns out.

You have the spring on there too right?? Spring goes on the needle, then the washer, then the oring. No extra orings down the hole? The needle baseline setting should be 1 3/4 turns counter clockwise from fully seated. You shouldn't crank down the needle too tight in the seated position but you should be able to feel the tips of each needle poking into the carb bore when seated. Also, when you bench synch you should gauge from the engine side of the carbs not the intake side. The slides milling is much more precise on that (non cutaway) side. This means you do it with the carbs off on your work "bench" and although there are a number of techniques, I find that a small drill bit between 1/16th and 1/32nd works well. You will of course have to adjust idle speed when you are done. Seems unlikely that this is your issue but perhaps . . .

eurban · « **Reply #11 on:** July 16, 2009, 10:00:11 am »

Two more things . . .When the choke is fully operated on PD carbs it bumps the throttle open slightly to keep the engine from dieing at idle when cold. When the choke is shut back off, this thottle bump goes away and you are left with the actual throttle screw setting. Is your throttle screw set properly? See if you can adjust it so the engine idles at 2500 rpm for starters. . . . Your typical 750 won't produce a voltage surplus at idle speeds. This means you are relying on your battery to keep things going. Hows your battery and charging system?

punch455 · « **Reply #12 on:** July 16, 2009, 10:49:08 am »

Yeah the needle is in there too. Sorry I forgot to mention that. The bench sync was done from the engine side, that's how I know the cutaways were NOT facing the engine. Used a 3/32 bit. As far as the choke issue, I'm in Chicago and the past few days have been around 85 degrees out. Haven't needed the choke at all. I'll adjust the throttle stop to try and get a 2500 steady idle just to perform some more basic tests. Since I've been working on the bike's idle, I knowingly have been working off a good 12V automotive battery and keeping the stock battery on the sidelines.

punch455 · « **Reply #13 on:** July 16, 2009, 03:55:14 pm »

POINTS!! I noticed that the color of my 1-4 points was just a shade darker than my 2-3 points! (I just installed new lighting in my garage) So, I grabbed some 120 grit and ran it through my 1-4 point, and VROOOOOOOOMMMMM budda budda budda. I was able to start tuning! Thanks everyone for the help. Forums win again.

hapakev · « **Reply #14 on:** July 16, 2009, 04:56:02 pm »

Quote from: punch455 on July 16, 2009, 10:49:08 am

Yeah the needle is in there too. Sorry I forgot to mention that. The bench sync was done from the engine side, that's how I know the cutaways were NOT facing the engine.

Aren't the cutaways supposed to be facing the engine?!?

hapakev · « **Reply #15 on:** July 16, 2009, 06:51:47 pm »

Quote from: eurban on July 16, 2009, 09:15:14 am

Also, when you bench synch you should gauge from the engine side of the carbs not the intake side. The slides milling is much more precise on that (non cutaway) side. This means you do it with the carbs off on your work "bench"

I mean isn't that what he is saying? anybody?

Thats the way mine are.

punch455 · « **Reply #16 on:** July 16, 2009, 07:00:19 pm »

I just read my own post again, and realized it's a bit confusing. When I wrote that I synced them on the side facing the engine, I meant facing the engine while the carbs were on the bike. (the engine was out too). I'm so sorry it came out totally backwards when I read it. The side of the slides that are NOT slotted is the same side that attaches to the air intake. Does that make more sense? Again, sorry about that.

BlackMax · « **Reply #17 on:** July 16, 2009, 08:29:40 pm »

The 3000 rpm then dies thing is odd. Makes me think there's an air leak somewhere....and a big one. You crank it and it has enough fuel or accel. pump boost to fire up, idle way high....but then go too lean and die. Air leak or restricted fuel supply?

How do the plugs look?

punch455 · « **Reply #18 on:** July 17, 2009, 07:35:05 am »

After I cleaned the point, it started up and idled fine. Started tuning it with the Morgan, and took it for a ride last night. It was a blast. I will have to go back in there with the Morgan and start adjusting a little here and there, but for a first ride of the summer.. WOWZA!! Thanks again. Heres a pic of me getting home.

hapakev · « **Reply #19 on:** July 17, 2009, 09:59:58 am »

Quote from: punch455 on July 16, 2009, 07:00:19 pm

I just read my own post again, and realized it's a bit confusing. When I wrote that I synced them on the side facing the engine, I meant facing the engine while the carbs were on the bike. (the engine was out too). I'm so sorry it came out totally backwards when I read it. The side of the slides that are NOT slotted is the same side that attaches to the air intake. Does that make more sense? Again, sorry about that.

No Worries, Glad to hear you got it running

eurban · « **Reply #20 on:** July 17, 2009, 02:08:43 pm »

Quote from: hapakev on July 16, 2009, 04:56:02 pm

Aren't the cutaways supposed to be facing the engine?!?

Seems to be some confusion here. No, the cutaways face the choke plate / air box. The "cutaway" refers arched profile on the bottom of one side of the slide. It does not refer to the notch in the slide. Different slides will have different cutaway profiles depending on the mixture requirements. For many carbs you can fine tune your mixture by purchasing slides that have richer or leaner cutaway profiles. For stock SOHC carbs we don't have this option but some will fine tune the profile with a grinder for mixture corrections . . .Glad to hear that you are making progress!

hapakev · « **Reply #21 on:** July 17, 2009, 05:26:54 pm »

The higher arches face the engine right? I think I remember seeing a small cutaway in the front, but I'm pretty sure that there are higher arches facing the inlet to the engine. Is this correct? Are my slides in backwards?!?

eurban · « **Reply #22 on:** July 17, 2009, 06:56:40 pm »

Quote from: hapakev on July 17, 2009, 05:26:54 pm

The higher arches face the engine right? I think I remember seeing a small cutaway in the front, but I'm pretty sure that there are higher arches facing the inlet to the engine. Is this correct? Are my slides in backwards?!?

You are either backwards or misremembering. The arched side (cutaway side) faces the choke plates / air filter. The relatively flat side faces the engine. If, for instance, you are doing a bench synch and are using a 1/16th inch drill bit as a gauging tool. You adjust the throttle stop screw and all the slides so that from the engine side you can just slip the bit into the gap between the carb bore and the bottom of the slide. Now if you take that same bit (with the slides / throttle stop still adjusted for the 1/16th bit inserted from the engine side) and bring round to the other (choke plate) side you will find that it is way to small. You might be able to fit a 1/4" bit between the bore and the slide. Got it??

hapakev · « **Reply #23 on:** July 17, 2009, 07:25:26 pm »

Thats exactly how I did it, so i must be cornfused

Thanks for the clarification

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Author Topic: AGRHHH Still won't idle! (Read 3410 times)