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high lift cam and ignition timing

Discussion in 'Chevy Trucks' started by Kyle, Jun 26, 2001.

  1. Kyle

    Kyle Junior Member
    Messages: 9

    I have a '78 GMC One ton dully and was wanting to time the ignition better than I have presently. It is a 350 with 400 sb ported heads with a quadrajet (aka: quadrajunk; dribblejet) carb and a mild lift cam with no increase in duration from stock. Obviously the stock timing would have been altered and I do not even know what stock timing was because when I bought the truck in 98 it had no information stickers under the hood and the engine was in the floarboard in rusted up peices.

    I was hoping to time it according to the vacume it produced as instructed by a napa store manager had told me to, but his method did not seem to work. He told me to use a vacume gage to find maximum vacume and then ****** the timing one dergree from that point with the timing light. I did this and my setting ended up off the scale at about 15 degrees advanced. Is it possible that this is correct, should I just time the engine to the standard 8 degrees advanced, or do any of you have better advice?

    I ask because my truck lacks the power it should have under heavy load when timed to 8 degrees advance and it seems to be timing related.
  2. 75

    75 PlowSite.com Addict
    Messages: 1,382

    Kyle - what are the symptoms when your engine is under load? Is it pinging/knocking like crazy, or just generally "down" on power? Just wondering why you're saying it's timing related.

    It doesn't sound like your engine is too radically different from stock, 15 degrees BTDC sounds like too much. I've got something fairly similar in my '75 dually, (350, Performer intake, RV grind cam and Q-jet) I find 6 degrees BTDC works well.

    Are the "quadrajunk/dribblejet" comments aimed at Q-jets in general or yours in particular? Mine runs fine and has plenty of power - within the limits of a small-block pushing an 8,000# truck. I'm wondering if your carb/fuel system needs some attention and possibly that's why your power is down? Is the rest of the ignition system up to speed? How is the engine mechanically? Has the lack of power always been a problem or did it just start, or gradually get worse over time?

    In any event, change only 1 thing at a time as you work at solving the problem. That way when you do get it figured out you'll know exactly what the "gremlin" was.
  3. raceman6135

    raceman6135 Member
    Messages: 61

    Hey Kyle, Rob (75) has some great questions you need to ask.

    As for the Quadrajunk comment, I also agree with Rob. A properly set up Q-jet is the closest thing you'll get to fuel injection in a carburetor. The small primaries will give you better fuel economy than any Holley ever will (and this is coming from a Holley fan!), and when those secondaries open up, it'll feed even a hungry 454 big block.

    In my '86 C25 crew cab with a 350 small block and automatic transmission, I run 10 degrees advance (measured w/the vacuum line to the distributor disconnected, of course -- you did disconnect and plug that line when you were setting your timing, didn't you?).

    I've also installed a Moroso advance curve kit using one heavy and one medium spring on the advance weights. This gives me a total of 37 degrees total advance, all in by 2900 rpm.

    Keep in mind, though, that I'm not towing an especially heavy load. My drag car and open trailer total only 4400 pounds. I also use 92 octane gasoline and there are only a few modifications done to my engine.

    General rule of thumb: run as much advance as you can without encountering detonation. Back off (******) the timing 2 degrees from the first signs of any detonation and keep an eye on your spark plugs (any black flecks that look like pepper on the plug tips is REALLY DANGEROUS! Or, if you see any small silver flecks, also BACK DOWN ON THE TIMING!)

    Have fun!
  4. Kyle

    Kyle Junior Member
    Messages: 9

    Sorry about the Q-jet joke, I was just kidding. I realize the relatively good nature of the Q-jet and that the cfm is hard to beat in anything that economical.

    Now down to the issue. I have since discovered that the timing was definitly the problem and I went with 17 degree advance (found with the helpfull hint from the napa store owner mentioned above). My cam is a C range lift cam so vacume is significantly altered and the degree on the cam also has an effect. As for the stockness of the engine, components are run of the mill but in my dino test it put around 300 hp to the ground at 4200 rpm. I forgot to tell you about the 9.8 to one compression. I was just timing the engine too slow, and early detonation is still not a problem with 17 degree advance. I only hear a little ping around 1100 rpm at full trottle.

    I was pulling 13 loads totaling 24,000 lb of round bales on a 5000 lb trailer with my 7600 lb truck so the power was definitly an issue. The first day of hauling I couldn't get out of 3rd gear and had to drop to 2nd in some of the grades. I had the problem fixed the second day of hauling and burned less than half fuel that I did before the reajustment and had no problem climbing the grades in high and sometimes 3rd gear.
  5. Kyle

    Kyle Junior Member
    Messages: 9

    missed a few points in my last reply. The engine is fresh, I did disonnect the vacume when timing, pinging was not a problem, i can run regular octane fuel without a problem, and i just thought about another possibility for the strange timing of my engine, the harmonic balancer may have slipped on the rubber giving the seeming erronious readings on timing. I will have to test my theory tomorrow. However the vacume method of timing seems to be dead on with the right setting, try it sometime! Thanks for all your suggestions, I appreciate it. Now that the hauling is done and I got to drive it without a trailer, the truck will realy scream!!!
  6. 75

    75 PlowSite.com Addict
    Messages: 1,382

    Kyle - those are some B-I-G numbers that "little 'ol 350" is pulling around! Good to hear that things seem to be working OK now.

    Wasn't sure about the Q-jet comments (if they were joking or not - hey, you forgot "Quadrabog! ;) ) so I figured I might as well ask. As far as all the other info, the more is known about the problem and symptoms the easier it is to come up with possible solution(s) when working "blind".

    How's your trailer set up with regard to axles/brakes etc? Just curious, I have a 5'th wheel flatbed trailer I built that comes in around 4,500#, tandem axles, electric brakes.
  7. Kyle

    Kyle Junior Member
    Messages: 9

    The trailer is coustom built with tandem duals it is a thirty foot goose neck style trailer with 14 inch chanel for the frame. It has electric over hydraulic brakes on both axles. This was done because the axles are extended ford f350 dually axles that still use the original hydraulic breaks and we wanted automatic engaging if something came loose so the power source is onboard the trailer. The truck needs more gears and I am looking for a non-overdrive, granny-gear 5 speed tranny like what was used in some 2 ton trucks of that time period. I also have my sites set on an electic shift two speed tailshaft to double my gears. The truck has been completely rebuilt from the ground up and I am not afraid to do it again. That's why I wanted to stick with the 350 engine due to the ease of getting parts for it and I figured if I didn't buzz it to hard and if I kept the output below a hp per ci I would get reasonable life out of it. I plan to add a inner shoe to the frame that extends from the leaf hangers to the front end of the truck for added strength in case of springs bottoming out.
  8. 75

    75 PlowSite.com Addict
    Messages: 1,382

    Thanks for the info Kyle - mine is 25' long, fifth wheel "highboy" style (basically a smaller version of the ones used in the oilfields)

    I've thought about building a trailer axle the way you describe, was wondering if I could find out some more about your electric over hydraulic brake system (make, model etc) ?

    The transmission you describe sounds like what's fitted on an old (1965! :eek: ) boom truck we have at my work. It's a C-60 with a 292 6-banger, 5 speed (L-1-2-3-4) and 2-speed axle. The transmission is a New Process, not sure of the model # but it doesn't look a whole lot bigger than the granny-low 4-speed in the '79 C-30 welding truck I drive.

    The "inner shoe" you plan to add to the frame - are you referring to doubling up the web (vertical part) of the frame? One of my friends is building a K-30 and did that using 3/16" plate cut to fit in manageable lengths, drilled/slotted to clear any rivet heads and then welded in place. Adds weight but that frame AIN'T gonna break!
  9. Eric ELM

    Eric ELM Husband, Father, Friend, Mentor, Angel
    from Chicago
    Messages: 239

    Rob asked me to post this picture for him. :)

  10. 75

    75 PlowSite.com Addict
    Messages: 1,382

    Thanks Eric! :) "Picture is worth 1000 words" or something like that...........

    The landing legs fold under so I can drop the front of the trailer down to ground level (winch & pole on truck) so no ramps required. Front & rear of the deck has a pipe "dummy roll" full width so the winch cable has a smooth surface to run against, deck is wood carriage-bolted down.

    Rigging box on the far side, and for the trailer plug I use a 7-pin big-truck style plug & pigtail.

    I hear you on the "truck has been completely rebuilt from the ground up........." Kyle - mine has been too (at least a couple of times!) and I still like to modify/improve it as I find time and come up with ideas.
    Last edited: Jul 3, 2001
  11. John DiMartino

    John DiMartino PlowSite.com Veteran
    Messages: 2,154

    Kyle the truck is underpowered,more so than needing more gears.You need a 2 ton truck to go along with the 2 ton tranny your looking for.Id check your frame around the goosneck regularly.At approx 35,000 GVWR you are almost 100% over what that truck was designed to do.The trailer axles,being F350 rears,are around7500 lbs max each,X2 =15000.your trailer weigh of 5000+ the 24K,is 29000.if 4000 is on the pin,your still almost 90% over the weight ratings of the axles,which you have made wider,which weakens them even more.You are really pushing your luck with that rig if ya know what I mean.My uncle has pulled 40Klbs with his old 84 1 ton for 15 years,about 6x a yr with no ill effects,you know how those farmers are.If the front wheels are still on the ground-pull it.If they are in the air,drop 5 suitcase tractor weights on the plow frame,and then pull it.
  12. reallyrusty

    reallyrusty Member
    Messages: 71

    just my two cents even though it sound like thing are more or less under control you might want to check the mechanical and vacuum advance parts on the your distributor.