carb problem after installing new cam
 

I have just installed a new andrews mild cam into my 91 ultra classic evo. The bike was running just fine before the mod.

Now the idle is rough and I can feel air popping through the carberator. Argggg!
I did a leak down test and air isn't leaking through the carb but is leaking through the exhaust.

What could be the issue here?

Thanks in advance for the help!

Ken

RE: carb problem after installing new cam
 

Without knowing the specs on the cam, and definitely not being a cam guru, I'll hazard a guess that the new cam has some/more overlap, meaning there is a period when both the intake and exhaust valves are open at the same time. This can cause some issues with reversion back through the carb. It can be exaggerated with a low restriction exhaust, which is the usual thing 'cause after all you are doing the cam to increase performance. I s'pose it'd be a good idea to make sure the cam was installed correctly 'cause if not, the timing would be affected, and could cause the rough idle & spitting, etc.

You could still have a couple other problems, and I'd wonder a bit about the air escaping past the exhaust valves during the leak down test. Had a leak down test been performed before the cam change? If not it's possible the valves were leaking before the cam change, and the new cam is just enough different that you're now getting enough reversion to affect things.

Also check for leaks around the intake manifold, especially where it meets the heads.

Just a couple thoughts; hope you get it figured out.

RE: carb problem after installing new cam
 

Thanks! Do you happen to know an easy way for a shade-tree mechanic to perform a leak down test? If so what exactly would I be concerned about as far as leakage is considered? How much pressure to put into the cylinder? Should it leak down at all? If so, how long would it take to leak down?

thanks again,

ken

RE: carb problem after installing new cam
 

I've never done a leak down test as I don't have the stuff to do it. Sorry I can't tell you any more than that about it.

I've usually just done the old "squirt the oil into the spark plug holes" to check whether it was rings or valves leaking by.

I think you'll have to make a plate that'd cover the intake, whether on front of the carb/throttle body in place of the air cleaner, or removing the carb/throttle body and putting the plate there. It'd need an air fitting, like a tire valve, I s'pose, and a place to put a gauge.

Anybody else know about this? What pressure, how much leakage/how much time?

RE: carb problem after installing new cam
 

For a leak down test. Found on a website its around 30-40 pounds of pressure into each cylinder. Though that wasn't for a motorcyle engine. It was for a small engine on lawn mowers and such. I'm not a mechanic, I'm just passing along some info I found. Hope that gives you an idea on how much air pressure.

RE: carb problem after installing new cam
 

kburks there could be a couple of problem's that first come to mind,it could be that you might not have the lash set properly on the exhaust valve's thus leaving them open a bit,also the carb will no dought will have to be retuned after a cam swap, what cam did you put in, this might help a bit more if we knew that.

RE: carb problem after installing new cam
 

First, I would recommend a compression test. This will let you know what is going on with the valves/timing. I would speculate that you could be out of time, possilbly one tooth on the cam. If your compression test is largely different between both cylinders this is more than likely the problem.

RE: carb problem after installing new cam
 

If you dont have a leakdown tester, you can perform a leak down by using a hose like that for a compression test and put air pressure on the cylinder while holding the piston for that cylinder at TDC. Holding it there will prove to be the problem, because the air will want to push down the piston. Once the air is applied you will know then if you are leaking from your intake or exhaust by listening to the air flow. Air pressure is not that big of a factor, keep in mind that a cylinder takes an explosion each time it fires.

RE: carb problem after installing new cam
 

My first question would be which cam? Did you get the biggest/baddest cam you could find? You may have over cammed ? Race cams run rough! You need a plan for all parts to go with each other...right cam with right exhaust and correct jetting etc. I would start with checking which cam you got and see if its compattable. If it is okay usually you need to do other mods to complament it, like jetting etc.:eek: Also like others have said, check the basics, compression etc.
Good Luck !

RE: carb problem after installing new cam
 

This will be long winded.

I do not know diddleigh about leak testing.

I put a cam in my 90 FXST last month.
Took two nights, because I sliced the gasket. Had a great time. This is therapy for me.

There is a great article in the new Amrican iron that talks about cams.
Also http://www.nightrider.com/biketech/hdstage2.htm has some great stuff

I have SE intake and V&H long shots. Stock CV carb jetted at 48 / 175 before the cam.

Before the cam I had popping and back fires on deceleration. Otherwise it ran really well.
I believe in getting the bike to where it is supposed to be with each mod. It is too hard to figure out when you do 4 mods and then try to figure out what it is supposed to act like.

I had a deaer parts guy suggest the SE4 cam. His eyes glazed over when he talked about the power. That is what sold me. It is the biggest cam I can use with stock heads. It si comparable to the EV27.

Anyway.
I put the cam in and the popping in the exhaust was gone.
It didn't sound or act like the same bike. very pleaseing. (maybe not to the neighbors)
It ran very lean.
My testing was wrong. I would jet re-jet and take it for a 2 mile run around the block. Nightrider suggests doing at a drag strip or big parking lot with multiple sets of new plugs. Getting the revs up in 5th gear and sutting the bike off and checking the plugs.
The way I did it just kept giving me lean plugs.
One of my earlier threads had a response that the plugs should be paperbag brown in color. This was my goal.
Finally I was jetted at 52 / 200. Still to lean, but better.
I took it out for 100 miles and checked the plugs. They were close to what the should be.

It also didn't idle worth a darn. Just bring the idle up a little higher. The taller cam is not going to idle like a stock cam. You will get some more engine noise also, because the ramps are steeper.

I remember the old muscle cars in the 70's that had cams. they would shake like crazy at an idle and sounded rough and run like a bat outta hell.

I am in SoCal. Dyno test is about $35 and a tune is about 200.
The HD dealer called the test a diag and wanted $95 and 200 to tune.
I figured since I am not running stock anymore I would go with the speed shop.

keep looking for articles. You are probably not that far off. The difference in sound and performace was a jolt when I first fired the bike up, but I can't stop smiling.