Backfiring when under load

[Wilms2004]

I thought I over filled the oil in my wifes 96 VS800. I put oil in it and the sight glass was too dark to see. Drove it about 2 miles home and then it started backfiring. Did a complete oil and filter change today along with new plugs and still having the same problem. It happens more when going up hill and accelerating. Idles and revs out fine when in neutral. What am I not seeing?

[WintrSol]

OK, is this a true backfire (coming out of the carb(s)), or afterfire (popping in the exhaust)? If the latter, check to see that the headers are tight.
Can you tell which side is doing it?

[Wilms2004]

I guess it is an afterfire. It seems to be coming from the right side. So off of the front cylinder. It feels like a misfire with an rpm interuption and a pop.

[Lechy]

When you checked the oil level did you put a block under the kickstand to get the bike as upright as possible then the level in the glass should be no more than 3/4 full.

[Wilms2004]

I added oil because I looked at the sight glass and the oil was low with the bike standing straight up. I rode the bike 2 miles or so after adding oil. It then sarted to pop and misfire. I assumed that maybe I had added too much. I got the bike home and checked the oil and the sight glass seemed to indicate that it was over full. I then checked the plugs and there seemed to be oil on the plug in the #2 cylinder. At that point I checked the air box for traces of oil. The air box did not show traces of oil. At that point I changed the oil and filter that it was due for anyway. I also changed the plugs. I filled with 3 quarts of oil per the manual and let it sit and settle. The oil level while on the jack was at the indicator line. I then went for a ride to see if I had solved the problem. At which point it was doing the same thing.... possibly worse than before.

[Lechy]

Might just need a good blow through. Get on the highway and give the old gal a good hard spanking.

[FallenAngel]

I might check sink
If it back fires sputters under load
Are you getting any smoke or oil out of the tail pipe??

[Wilms2004]

The harder I get on her the worse it seems to behave. As far as oil or smoke out of the tail pipe definitely no oil and as far as I have noticed no smoke either.

[FallenAngel]

The harder I get on her the worse it seems to behave. As far as oil or smoke out of the tail pipe definitely no oil and as far as I have noticed no smoke either.

If your not getting any oil or smoke from your exhaust then I don't think oil is your problem
I think it has been an ongoing problem of the cables stretching that you just noticed after adding oil. I could be wrong . But if it starts and Idles OK but back fires under load and there really is no Timing spec to speak of that leaves a carb problem

Try This
Start the bike and slowly roll on the throttle with NO LOAD. Is it smooth???
Then try it with a load
Slowly roll on the throttle If it back fires slowly roll off of the throttle keeping it under load if it smooths out and gains speed you have a sync problem
I would Check your sink

Here are 2 Do it yourself videos
You only need 2 lines for the Intruder
I just like this one because it just looks cool


Vacuum gauges from amazon
Should be a synced set ( Do not drop one of these)

This one is far superior to the gauges and iI think you can get it at Amazon
https://www.ebay.com/itm/274715693342?c ... wqEALw_wcB

The Home made versions work just as well and can be assembled for just a few dollars
There used to be a lot of Carb sync info here but seems we have become unworthy and posts have been deleted

[Lechy]

Here is a link to the carb synch thread I posted.
http://intruders-alert.com/viewtopic.php?f=32&t=4139

[FallenAngel]

Here is a link to the carb synch thread I posted.
http://intruders-alert.com/viewtopic.php?f=32&t=4139

And thats team work
Thanks Leachy

[Herb]

The kind of problem you are having seems to be a lean condition under load. Possible plugged up main jet or even a problem with the slide not working right. could also be an issue with getting enough air through the filter.

[FallenAngel]

The kind of problem you are having seems to be a lean condition under load. Possible plugged up main jet or even a problem with the slide not working right. could also be an issue with getting enough air through the filter.

I lean condition should reveal itself with a slow roll on the throttle
It will rev slow stumble and back fire
An extreme condition will be hard starting and a miss fire on the offending cylinder with surging when the offending cylinder fires

[Herb]

The kind of problem you are having seems to be a lean condition under load. Possible plugged up main jet or even a problem with the slide not working right. could also be an issue with getting enough air through the filter.

I lean condition should reveal itself with a slow roll on the throttle
It will rev slow stumble and back fire
An extreme condition will be hard starting and a miss fire on the offending cylinder with surging when the offending cylinder fires

When the rear cylinder slide diaphragm on my 87 went bad (had a 1/4in tear in it) the rear cylinder on it did the same thing. Not sure but I think the slide was moving up and down with fluctuating vacuum. It ran great without a load but on the street, it did all kinds of weird shit. replaced the slide and the problem went away. I have also seen a partially plugged jet cause this kind of thing.

Had a friend whose 1958 Ford Fairlane with the 332 and a 4 bbl carb was running great and then started bucking and jumping when he got on the throttle hard, it would also backfire from time to time. It ran fine if you walked on it easy or sitting in the yard with the hood up. We fought it for over a week and then we found that the front part of the underhood insulation had come loose and when he got on it hard it would suck the insulation against the air horn on the air filter. We discovered that when we pulled the cover off the air filter and found bits of insulation on the filter.

My dad had a 1960 Chevy pickup that was doing that kind of thing, but only part of the time. Turned out the bronze fuel filter to the carb was almost completely plugged, replace the filter and the problem went away.

All kinds of things can cause this kind of problem.

[Lechy]

I remember that old 750 of mine giving me a similar problem, I spent a week working on the carbs without success scratched me a couple of bald patches on my head. I checked the battery terminals. cleaned them all up. fitted star washers but still had that effing missfire. I did notice that the earth lead was not flexible for about 2" at the battery so I figured that the inner cable had heat hardened and was causing a high resistance. Got a bit of cable and a couple of lugs and made up a new earth cable, even soldered the lugs up. All problems miraculously disappeared. Vroom Vroom.

[Wilms2004]

I really appreciate all of the ideas guys. I am going to try these things out for sure. I tried slow rolling the throttle yesterday. The engine would completely cut out and then i would get a loud sharp pop out of the exhaust after which it would go for a few more seconds and repeat. It completely died on me when I pulled the clutch to slow down but fired right back up. That has never happend to me yet.

[Lechy]

Did you check the condition of the air filters? Gummed up filters can cause misfires also li'l critters love to nest in the air boxes and eat the filter paper.

[Wilms2004]

Did you check the condition of the air filters? Gummed up filters can cause misfires also li'l critters love to nest in the air boxes and eat the filter paper.

I have new filters ordered. The bike is new to us so I am slowly going through it. It ran good um until I put oil in it. Probably a coincidence but won't know til I figure it out.

[FallenAngel]

Did you check the condition of the air filters? Gummed up filters can cause misfires also li'l critters love to nest in the air boxes and eat the filter paper.

I have new filters ordered. The bike is new to us so I am slowly going through it. It ran good um until I put oil in it. Probably a coincidence but won't know til I figure it out.

Does seem a bit odd coincidence or not.
I am going to assume that it got progressively worse
May I suggest you look for a vacuum leak
If you need suggestions on how just ask
I myself use propane and a small hose with engine running

[Lechy]

New to The Intruders. OK hope you have a lot of patience when working on these machines. When you replace the air filters it is imperative that the boots from air boxes to carbs are correctly seated (first thing to do after removing them would be give them a good inspection for cracks, splits and tears). The front one isn't too bad to get right but the rear can be a royal pain in the arse. I found it helps a lot to remove the 3 fixing bolts on the top of the air box, 2 outboard RH side 1 inboard LH side then loosen the 2 clamps, push the airbox back a little and pull out the boot. When re-fitting, spray some WD 40 around the airbox hole, push the box back and fit the boot to the box before lining up on the carb and pushing the box back into place. The hole on the airbox has a lip which causes all the trouble getting the boot located. One of these is invaluable to achieve this end:


Good luck.