Topic: intake whomp?

[notarex]

I'm trying to get my 74 cb550 sorted out and it's been going well. It runs like a top on short trips, so I decided to take a longer trip to see how it felt. About 8 miles into the ride I felt a noticable loss of power, and when i rolled on the throttle all I got was a WHOMP through the intake and it stalled. I pulled over, checked the gas flow, air filter was tight, battery charged everything looked ok. let it cool for about 10 min, made it about 5 miles more, same thing happened. Long story short, I made it home but am concerned and disappointed. Any ideas what the problem could be?

[Jerry Rxman Griffin aka MuthaF'er]

Sounds like your electrical/ignition system shut down, cooled off, and restarted. I think if it were the fuel system it would have started leaning out on you rather than whomping you. Wish I could be more help.
Jerry

[TwoTired]

Do you know if your fuel tank vent is clear in the gas cap?

Things that can restrict fuel flow:
 - Rust/scale in the tank
 - Rust/scale in the petcock screen
 - pinched/ collapsed deteriorated fuel line.

Other thoughts:
 - Float level set too low.
 - bowl vents or overflow tubes blocked.
 - is your choke staying in position?

Have you had a full, complete tuneup in the last 3mos. or 3000 miles?

What modification from stock have been done to the bike?

[notarex]

Well the tanks not perfect but I have filters on both the lines which seem to be flowing well. Petcock screen will need to be checked. I set the floats per the manual, overflow and vents are clear, choke is functioning properly. Plugs are new, points are newish, valves and point gap adjusted recently. Timing not done recently but it runs fine until this happens so I haven't messed w/ it. As far as modifications it has a 550SS 4-1 exhaust and a custom airbox using the stock box with a fabricated block off plate and adapter to mount a K+N designed for a suzuki 550. It doesn't seem to be running lean judging by the plugs which are nesquicky good.

[TwoTired]

Do you know if your fuel tank vent is clear in the gas cap?

Check this?

Well the tanks not perfect but I have filters on both the lines which seem to be flowing well.

Any loops in the fuel lines that can create air pockets in the gravity feed system?  The fuel tank must be the only high point in the feed system.
Do you know the lines are flowing well "in situ"?

I'm guessing the "whomp" is an intake backfire from too lean a mixture (fuel starvation) coupled with the advanced timing due to the spark advancer (normal).

Did you set the intake valve clearance cold? (0.002 in.)

Cheers,

[notarex]

Thanks two tired, I feel like a moron- the petcock screen was full of rust. The motor was probably sucking enough out to go for a while but it would not flow on the bench.  Looks like I have some kreem or por15 in my future! I drained and strained the gas for now and stripped and cleaned the petcock. BTW I have my fuel lines routed low and in a curve on the left side to fit the fuel filter, meaning there is a point on the fuel line that is lower than the carbs. Is this bad? I did this to avoid a kink in the line, but if I found a 1/4" 90 degree barbed adapter i could avoid this. Should I look for one?

[TwoTired]

The motor was probably sucking enough out to go for a while but it would not flow on the bench. 

Clarification:
The motor only sucks the fuel out of the carb bowls.  Gravity draws the fuel from the fuel tank.

BTW I have my fuel lines routed low and in a curve on the left side to fit the fuel filter, meaning there is a point on the fuel line that is lower than the carbs. Is this bad?

No direct experience, here.  But, others have reported fuel delivery issues with looped lines.
The float valve is held closed with the float while the bowl in full.  But, either gravity or head pressure from the fuel above it pushes it open.  If an air pocket forms on the up side of the valve, and there is any resistance to valve movement, gavity may not let the valve fall and this will prevent the bowl from filling.  Any plumber will tell you that a drain pipe must have a consistent and steady drop along it's length to flow properly.  I believe the tank to carb hose should be similarly plumbed, as it is on the stock bike.  It's a gravity feed system.

Cheers,