Author Topic: engine bogs down  (Read 353 times)

briandneville

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engine bogs down
« on: July 09, 2022, 02:39:01 PM »
Rebuilt a set of carbs from a friend's '83 and installed them in my '83 to test.  Synched and idle mix set, idle adjusted, all seems fine in the driveway, except that there is a slight backfire audible at idle.  Taken it out for two test rides and it just does't run right.  Bogs down when throttle is opened up 1/3 to 1/2, at just about any rpm and never has the power or pull I am used to (with my carbs).

In rebuilding my friend's carbs, I used the same process I went through with mine a year ago: Keyster kits, ultrasonic cleaning, replacing all diaphragms possible, float valves, blew out all passages, verified fuel flows through the bowls as expected,accel pump works and streams flow properly from nozzles, installed jets that came with the kits, new fuel lines and no leaks present.

I did not check the float bowl levels, and I did not replace the butterfly valve seals.  I am fairly sure that I have all vacuum lines properly connected with no leakage.

It seems like I may be missing something obvious but I cannot think of anything.  Any ideas would be appreciated!
How did it come to this?

fret not

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Re: engine bogs down
« Reply #1 on: July 10, 2022, 03:07:42 AM »
Just offering some ideas:

I suspect the jets in the kit.  Unless jets are altered they should not wear, therefore, if they worked previously I would try the original ones again.
 
Backfiring??  Suspect cam timing or valve clearances or lean mixture.  Have you changed any of this recently?

Battery fully charged?

We will be curious to know how this is rectified.
Retired, on the downhill slide. . . . . . . . still feels like going uphill!

briandneville

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Re: engine bogs down
« Reply #2 on: July 10, 2022, 10:36:53 AM »
The carbs in my bike now (not mine) came out of a vision that has not run in 10-15 years, so there is no way of knowing how they worked last, but I agree with the logic behind your comment about the jets.

Backfiring is odd, as it was not doing that with my carbs in it, and yes it suggests a lean mixture, which would indicate airflow issues, so maybe the air jets?

Battery is brand new, fully charged, BUT I did replace it at the same time that I took my carbs out and put this other set in, so who knows?

I will add comments attempt to sort this out. Will start taking things apart and looking for bonehead moves, slowly working towards taking the carbs out and putting mine back in to verify the problem is the carbs and not something else. 
How did it come to this?

jefferson

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Re: engine bogs down
« Reply #3 on: July 11, 2022, 09:29:55 AM »
I'm in agreeance with Fret Not on the jets. The orifices don't go bad they just get plugged up or decreased in size due to varnish. Clean them good and reinstall. I only use genuine Mikuni jets.

fret not

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Re: engine bogs down
« Reply #4 on: July 12, 2022, 02:32:04 PM »
Another possibility is:
   the transistor ignition box.  They have known issues due to years of 'exposure'. (PC board deterioration at edges, etc.).
  air leaks in the carb boots (they tend to form cracks as years paaa)
   as previously mentioned, the butterfly shaft seals
Retired, on the downhill slide. . . . . . . . still feels like going uphill!

jefferson

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Re: engine bogs down
« Reply #5 on: July 12, 2022, 03:21:44 PM »
If your bike ran fine with your carbs on it then the difference is the new carb set. Spray some carb cleaner around the butterfly shafts with the bike running and see if that does anything. The manifolds could also become more stressed with removing and installation of the carbs.

briandneville

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Re: engine bogs down
« Reply #6 on: July 13, 2022, 10:14:29 AM »
Found that the main jets I installed from the rebuild kit were 125 and the spec is for 130 so I put the old ones back in (they are clean, slightly tarnished, and one of them has a somewhat mangled top, but they went in fine). 

I am suspicious of the pilot air jets, given that the kit did not have the correct main, but haven't taken th tank and airbox off to verify; possibly these are too small and choking off the air at idle, but seems like the mixture screw adjustment would resolve that, but maybe not, and maybe this is the reason for the mild backfire at idle.  I will have to dismantle and assess.

My carb boots were new when I installed them last year and don't appear to be defective or suspect.

The TCI is always a suspect given that it is 40 years old but as I said the bike ran great with my carbs in it so I will ignore that for now.  I have a backup TCI, though it's also 40 years old.

With the shaft seals / carb spray:  is the idea that if the seals are bad, the engine will intake from the shaft area and that will increase idle speed, or affect idle in some way?  I will give that try if the air jet thing doesn't pan out.  I will also verify that the airbox flapper is ding what it is supposed to do. 

Thanks for your help.
How did it come to this?

briandneville

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Re: engine bogs down
« Reply #7 on: July 17, 2022, 08:25:41 PM »
Pilot air jets were too small, so I put the old ones back in.  The front carb accelerator nozzle was clogged and no fuel was getting through.  I suspect this was the main culprit, as the engine ran nearly as well as it should at higher rpms when I tested it out.  Idle is all wrong now so I need to re-synch and adjust the mixture screws correctly.  It also backfired when trying a hot re-start.  But it did restart.

I think I am getting there.
How did it come to this?

fret not

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Re: engine bogs down
« Reply #8 on: July 18, 2022, 03:59:49 PM »
Brian, have you checked the valve clearances?  As this system  wears  the clearances get tighter, even to the point that valves don't close completely. (one possible source of backfire)
Retired, on the downhill slide. . . . . . . . still feels like going uphill!

briandneville

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Re: engine bogs down
« Reply #9 on: July 19, 2022, 11:46:15 AM »
from what I've heard and read, checking valve clearances is not something one does in one's spare 15 minutes, so no I have not done that...BUT I bought the bike from Fiat Doctor who adjusted them and felt it would be good to 50k (it has 34k on it now) so I am not thinking that I need to do go that level yet.  I never had a backfire when my carbs were in the bike (I am looking forward to putting mine back in!).  More later the there is more to tell.
How did it come to this?

fret not

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Re: engine bogs down
« Reply #10 on: July 24, 2022, 02:08:02 AM »
Brian, checking the valve clearances is not a picnic but is not terribly difficult.  It becomes difficult when you need to change some shims.  My point about checking the clearances is to verify none are too tight, which could be a cause for backfire.  You probably have the solution in your hands with your carbs.
Retired, on the downhill slide. . . . . . . . still feels like going uphill!

briandneville

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Re: engine bogs down
« Reply #11 on: August 07, 2022, 01:25:56 PM »
Went back through the carbs and found a number of things to correct:  accel nozzle plugged on front carb, pilot air and main jets incorrect size, idle synch so far out I could not even begin to synch until I visually adjusted the butterflys to the same position.  It now runs smoothly in the driveway, starts cold and hot, idles great with no backfire, and is fine under normal city riding loads.  BUT it has a lag upon moderate or heavy throttle, in the 4-6k range but also seems to be present at much higher rpm.  Seems like something is not right with the main fuel pathway in these carbs.
 
Going to remove them and put my carbs back in to verify that this lag is attributable to the carbs I've been rebuilding for a friend, and not to something else with my bike. I've never experienced any sort of lag, bog or stumble with this motorcycle previously so am pretty confident that these symptoms are caused by something in this set of carbs.

More when I have something to report!
How did it come to this?

fret not

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Re: engine bogs down
« Reply #12 on: August 08, 2022, 02:36:27 AM »
It's great to see some progress happening, Brian!  We are all curious to hear how this sorts out.
I wish I had time to work on MY bike, as it is closer to actually operating but not yet ready.  Fuel system to plumb yet, and a loop in the cooling system for the carbs to prevent icing.  I had never seen anything like this but it is a standard feature on the ThunderCat where the carbs came from.  I'm still cutting and dragging brush to be chipped from the uprooted trees that came down in late December and doing my days trying to mitigate the fire danger.  If this area burns it will be huge because of all the 'fuel' on the ground that is now rather dry.  I am always on edge this time of year until we get some significant rain, and this year is a bit more stressful due to the unusual load of fuels.  Hoping for rain.
Retired, on the downhill slide. . . . . . . . still feels like going uphill!

pullshocks

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Re: engine bogs down
« Reply #13 on: August 10, 2022, 03:20:29 AM »
My Vision was sold 8 years ago, but the painful memories remain of chasing carb problems, even to the point of replacing the butterfly shaft gaskets.  In the situation you describe, I wonder if one of the carbs still has crud in the pilot jet circuit leading to the tiny holes below the butterfly.  In that condition you can get it to idle but will experience severe bogging when you roll on the throttle.

The pilot circuit is VERY hard to clean,  I think I used my soft tip compressed air gun on the tiny holes to blow in the opposite direction from normal fuel flow, after injecting spray carb cleaner in there as best I could with the little red tube.  I might have used some  tooth brush bristles or something in the tiny holes

I started a thread about this episode ....http://ridersofvision.net/rovforum/index.php?topic=13359.msg121120#msg121120

Sounds like you have done some good work on those carbs including the all important accelerator pump nozzles.  Your friend is lucky indeed.

I still look forward to getting a look at your restoration job one of these days.

Pullshocks/Mark