Thats some really good points P51D, I have a AotoMeter Air/Fuel mixture gauge that works off of a weld in O2 sensor in the exhaust stream, it may be something to consider for monitering the mixture and injector sizing or jetting requirements.

 

this vehicle was and is on propane which is very hard on valve seats (exh) and I'm confident the mixture was close from the colour of the plugs, but you have made an interesting point although it feels like it is bogging at low rpm under load and if the throttle is feathered carefully with out giving the engine too much throttle it will pull away and once higher rpms are reached it pulls strongly. I have adv. the timing considerably and moved the curve to a much faster adv. which has helped significantly, propane likes adv. up to a point.

 

It sure sounds like a timing issue, did you remove or replace the timing chain assembly while working on it?

 

no changes there in fact never took the timing cover off. adv. the initial timing and recurved the dist to adv. sooner and it helped a lot although I still have to feather it off the line but it's better.