Hi Idle when Warm (Long, first post, sorry) :)

Greetings all.

I have a recently aquired '81 SECA 750 that I have been working to get back into shape. This site has been incredible for finding information and has helped me get the bike on the road. There are, however, a few items I am still dealing with. The main issue that I am looking for suggestions on is the good old High Idle when Warm!

I think I have read about every post on the subject and while my situation is similar to most, I can't find an exact match to my symptom.

Symptom => Bike starts up right away on "choke", I lean it off of that and take off on my ride. Go down the road 10-20 mins bring in the clutch and rpms usualy don't go below 3k. (Bring the clutch in and they don't go down much) I can adjust the idle screw (big star in the center of the carb rack) down to normal idle, but then it wants to idle below 900 and die. I can tweak it back in to 1k+ and it holds till I blip the throttle...revs up and then drops down (usualy slowly depending on how low I have the idle screw set at) hangs at that 1,1k mark or so and then slowly drops down.
Performance on the road is fine.

What I have done => First thing i looked for was air leaks. Tried WD-40 anc carb cleaner in all the clasic and recomended spots looking for that rev up...nothing. Took a look at the boots and RTV'd any cracks or possible areas of leackage. No joy. Thought maybe it was becuase they were way out of sync. Got my fancy new "Carbtune" set this week and synced em up...runs better but still the same issue. Got my cans of spray out again (after RTV'ing the boots) and hit the boots, vac nozzels, diaphram covers and hinges...no RPM rise that I could detect. I have also checked to make sure that none of the carbs are hanging up...there is no binding or hanging up on the throttle cable either.

Preliminary conclusions => A vaccum leak that I just have not detected. I read about using propane to find the leak and that seems like a good way to do it. I will try that tomorrow when I have access to propane and see if I can find a leak that way. Mixture possibly? I took a look and what do ya know, the "anti tamper" pluggs are still in! Got a color tune plug that will probably arrive tomorrow, so, need to drill those out carefully (have the good tools for that) but if the mixtures have never been messed with, could they be that far off?
I'm looking for anything else i may have missed. Going down the road, or holding an RPM over say 2k and there is no fluctuation, power holds steady. No I have not cleaned the carbs yet (except for running seafoam through em). I suppose throttle shaft seals could be part of the problem as the ones in the middle are kinda hard to get to with the tank on. A hole in the diaphram maybe?

As stated I am guessing that when I put the propane to it, I will find the leak that has been avoiding me...though I am worried that it will take more than just RTV to fix it and I would prefer something simpler. Oh, testing this on a full tank of gas and have run it on both prime and on so that also should be an issue. (not that it would be). Also, I do get mild backfire sometimes when running at higher rpm with no load or throttling down. (very mild) (mixture too rich?)

There I think I have covered all the bases on that one. Plenty of other questions, but this is the one that I am focusing my attention on for now..(and this post is already long enough!)

Thank you for your patience and advice!

-Geej


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'81 Seca 750

 

Quote:Thought maybe it was becuase they were way out of sync. Got my fancy new "Carbtune" set this week and synced em up...runs better but still the same issue...

Did you use a YICS tool when you synced? If not set your idle fast (1400 RPM) to sync without the tool or try Rick's clothes line Mavel Mystery Oil YICS tool.

Syncing is a fine art, very subtle movements of the adjusting screws are required.

Mixtures might be off (a bit) or carbs dirty? Checked float levels? Has the bike been sitting for a long time?

Sync-mixtures-sync-mixtures and maybe sync-mixtures again! You are fine tuning the motor here!

If the WD 40 did not find a leak there probably is not one? Soaked (not just a little squirt!) with WD 40 from the "extension" squirty tube on the can, you should be able to find a leak.

BE BLOODY CAREFUL with propane!

You must be close to getting the bike running right, with these older machines it can sometimes be a case of going over everything, carbs, coils, plugs, loom, valve shims, replace the fuse box untill you get to the point it can't be anything else and all of a sudden Vrooooommmmmmmm!

Good luck

 

You need to Clean-Tune the bores that the Diaphragm Pistons travel up and down in.
A Diaphragm Assembly that is slow to respond to changes of the throttle is going to be responsible for a host of performance issues including the situation about which you write.

Search: "Clunk Test" and take the necessary steps to insure that there is adequate movement of the Diaphragm Assembly's during all throttle conditions.

 

Ah yes, I knew I left out something.
NO I did not use the YICS tool, though I did sync them a an idle closer to 1500 for that reason.
I read about and will try another sync with the clothes line tool.

Yes, I can imagine that you need to be extra careful with propane...maybe i don't need to try that? Um, yes, SOAKED all possible areas with wd-40 and notin.

I'll read up on the "clunk test". What would be symptoms of a hole in a diaphram? And would you hear anything? there are times during start that I hear a diaphramish squeak (if that makes sense) kind of like the sound you might hear from a diaphram pump. Could that be a hole in a diaphram? Of cousre, again soaked the diaphram covers in wd-40 and no rise. Could try it again though.

I have not checked the valves. The fuse box is all new as are the plugs. Have not checked float levels. Mixtures are likely off some...but would off mixtures do this? The bike did sit for a bit...one of those (it ran last season) stories. I was hoping to get through this riding season without yanking the carbs and then this winter take care of the carbs and other non-urgent issues. As stated the bike starts right up every time and runs strong on the road. Right now I have the idle screw set so that the idle does not hang hi...but at stops I have to have just a little pressure on the throttle to keep it from cutting out.

Ah, when I get the mixture covers off is there any way of cleaning out that idle circut without taking the bowls off?

Thanks again!

 

Just finished the "clunk test" on my carbs fully expecting to find gummed up or grabbing pistons and/or holes in the diaphrams...
Nope. All clean, no holes and the pistons drop like a rock. Go clunk big time. Act like a guiotine (sp?) etc, etc. That part of the carb was pretty clean, needles look good enough to sew with...Ok, enough of that...

I took a look at the plugs while I had the tank off...I'm no great plug reader, but I have an idea. The mixtures on 1 and 4 seem to be the same, 2 and 3 are different from each other and 1 and 4. Not much, but I think a bit cooler, while the outside ones might be hotter. I'm not even going to try to explain how they looked or go further as I plan to colortune asap.

Other ideas? of course, I still have not torn down the carbs, adjusted mixture, checked the valves or the float levels. I did not put propane to it yet either (and may not at all...advice?)

Thank you for your help!
-Geej

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'81 Seca 750