Exhasts sucks as well as blows???

[1340evo]

Thats interesting.. no idea why I did this.. other than I have a vibration on my 1340 that I'm trying to find... was trying to time it up using a timing light but it works on the rear lead, but not on the front?... keep the leads the same on the coil, but switch the front to the back, and back to the front on the plugs... it still onlt works on the rear lead (that was the front before... ) so plug???... switch the plugs... nothing changes???... ...
anyway, thats not why I'm posting...

I have V&H pipes on it... i covered the front pipe with my hand... and at constant revs it blows my hand off when it fires... do the same on the rear, it blows my hand off, but I can also feel it suck back on?... rear pot always runs richer than the front... it was built by the bloke I bought it off... any idea whats out?... could the cam be out a tooth??.. or is it just a push rod issue do you think??... if it was holding open on the rear you'd expect it to run lean I'd of thought???..  and compression test see them both the same??  all very strange.

[1340evo]

would it run a tooth out?... just wondering if its a slight twist in the crank... keyways worn... crank gear etc all one way or worn so showing in line but actual a few deg out?... would it make this differance or am I looking for something much bigger?... how many teeth out will they run for instance?
I'm used to dailing cams in to get the cock on... I'm guessing there is no adjustment on these??..

[K4FXD]

I'm not familiar with that pipe, but are the front and rear the same length?

You may be feeling reversion. Change the RPM and see if the front starts sucking too.

All I got.

[1340evo]

Same length... been thinking whilst at the pub.. if it was cam.. it would do it on both.. so push rods or breather timing I'm thinking?

[rigidthumper]

reversion combined with cam timing events and a shared intake makes the rear more pronounced- look at what one cylinder is doing while the other one is in overlap time.

[1340evo]

OK.. but what are you saying.. this is normal?....

[rigidthumper]

Very common, and more noticeable with open pipes.

[1340evo]

ok.. so does it contribute to vibration?... should I be looking to correct it?... if so how?.. the pipes are not joined... the rear cylinder runs richer than the front.. Is this common?
If so I'll let it be.. if there is anything to investigate I'm in there?... any pointers?

Cheers...

[rigidthumper]

Not an issue. Some vibration is normal and expected, as you have a solid mount engine that transmits pulses to the frame. Two pistons on a single connecting rod, that throws forces up/down with every explosion of the fuel, and that condition can be aggravated by incorrect timing, fuel mix, components in the engine, etc. Apehangers make it more noticeable in the arms, chopper style seats transfer more pulses to the posterior, solid pegs move it into the feet. That's why thick seats, and cushy grips, and rubber floorboards are popular.  Most old timers switched to rubber mount baggers, or counterbalanced (Twin Cam) Softails, so they could spend more time riding...

[jbexeter]

Most old timers switched to rubber mount baggers, or counterbalanced (Twin Cam) Softails, so they could spend more time riding...

Amen...

Due to the basic design of the engine it's *very* to hard to make *all* conditions *identical* or as near as for both cylinders.

With old BSA pre unit twins running a single carb you could use wedge shaped shims to point a carb more one way that the other to adjust the mixture, back in the shovel / su days we'd always file down *only* the one branch of the intake manifold to try and do the same thing, and it seemed to change from motor/carb/filter to motor/carb/filter which side you'd have to do to lean up the rear.

Don't forget exhaust gas isn't just a pumped volume of gas, it's also cooling, which changes volume and pressure  and front and rear have different pipe geometry, put two pipes that go out horizontally to the right with only one sharp bend at the head so they really are identical pipes and some of it will go away.

put your ear to the end of the pipe or intake manifold and you will *clearly* hear a valve that is blowing so (exhaust gasket leaks aside) if the valve is seating then everything else that is happening is a function of the exhaust pulse in the pipe.

don't forget that (generally speaking for all engines) a valve that seats well and seals well might still have issues that cranking compression and leak down tests won't find, like a broken spring... it's work nearly perfectly at cranking and tickover rpm..

[1340evo]

Up the RPM and it all comes together... both at 170 PSI on the nail... so guess its OK.. just curious about the pulling back bit?... as you say, no pipes are exactly the same... and even though they are the same length with 2 bends in each, I'm sure it depends where the bends are as to how they both perform....
BTW it's a FXR so rubber mounted.. but TBH I'm thinking this adds to the vi9bes instead of improving it LOL... I'm sure my old Shovel was better??... Fornt end is fine.. more seat... guess its more padding required.... as it is quite hard...

[K4FXD]

Not sure about the FXR's but the Dynas will vibrate your teeth out when the motor mounts wear out.